Add example mtr prober from `mtrpacket`

Started rejigging example code from this example to use more modern
`asyncio` APIs:
https://github.com/matt-kimball/mtr-packet-python/blob/master/examples/trace-concurrent.py

Relates to #330

scripts/mtr.py

@@ -0,0 +1,196 @@
+import asyncio
+import curses
+import itertools
+import sys
+
+import mtrpacket
+
+#
+#  ProbeRecord keeps a record of round-trip times of probes and repsonder
+#  IP addresses, for a particular time-to-live (TTL) value.
+#
+#  There may be multiple IP addresses for one particular TTL value,
+#  because some networks have multiple equally weighted routes.
+#
+class ProbeRecord:
+    def __init__(self, ttl):
+        self.ttl = ttl
+        self.success = False
+        self.ip_addrs = []
+        self.probe_times = []
+
+    #  Format the information about this line for display
+    def print(self, screen):
+        line = '{:>2}. '.format(self.ttl)
+
+        if self.ip_addrs:
+            line += '{:42}'.format(self.ip_addrs[0])
+        else:
+            line += '{:42}'.format('  ???')
+
+        for time in self.probe_times:
+            if time is None:
+                line += '          *'
+            else:
+                line += '  {:>7.3f}ms'.format(time)
+
+        #  Use curses to display the line
+        screen.addstr(line + '\n')
+
+        #  List IP addresses beyond the first
+        for addr in self.ip_addrs[1:]:
+            screen.addstr('    ' + addr + '\n')
+
+
+#  When we've got a result for one of our probes, we'll regenerate
+#  the screen output, and allow curses to refresh it.
+def redraw(screen, hostname, all_records):
+    screen.erase()
+
+    screen.addstr('Tracing to "{}"\n\n'.format(hostname))
+
+    for record in all_records:
+        record.print(screen)
+
+        #  If one of our probes has arrived at the destination IP,
+        #  we don't need to display further hops
+        if record.success:
+            break
+
+    # screen.addstr('\n(press SPACEBAR to exit)\n')
+
+    screen.refresh()
+
+
+#  Perform multiple probes with a specific time to live (TTL) value
+async def probe_ttl(
+    mtr,
+    hostname,
+    ttl,
+    record,
+    redraw_callback,
+):
+    for _ in itertools.count():
+        result = await mtr.probe(hostname, ttl=ttl, timeout=6)
+
+        if result.success:
+            record.success = True
+
+        #  Record the time of the latest probe
+        record.probe_times.append(result.time_ms)
+
+        addr = result.responder
+        #  If the address of the responder isn't already in the list
+        #  of addresses responding at this TTL, add it
+        if addr and addr not in record.ip_addrs:
+            record.ip_addrs.append(addr)
+
+        #  Redraw the display, which will include this probe
+        redraw_callback()
+
+        #  Wait a small amount of time before sending the next probe
+        #  to get an independent sample of network conditions
+        await asyncio.sleep(0.1)
+
+
+#  Launch all the probes for the trace.
+#  We'll use a separate coroutine (probe_ttl) for each ttl value,
+#  and those coroutines will run concurrently.
+async def launch_probes(screen, hostname):
+    all_records = []
+
+    #  When one of the probes has a result to display, we'll use
+    #  this callback to display it
+    def redraw_hops():
+        redraw(screen, hostname, all_records)
+
+    async with mtrpacket.MtrPacket() as mtr:
+        probe_tasks = []
+
+        try:
+            for ttl in range(1, 32):
+                #  We need a new ProbeRecord for each ttl value
+                record = ProbeRecord(ttl)
+                all_records.append(record)
+
+                #  Start a new asyncio task for this probe
+                probe_coro = probe_ttl(
+                    mtr,
+                    hostname,
+                    ttl,
+                    record,
+                    redraw_hops,
+                )
+                probe_tasks.append(asyncio.ensure_future(probe_coro))
+
+                #  Give each probe a slight delay to avoid flooding
+                #  the network interface, which might perturb the
+                #  results
+                await asyncio.sleep(0.05)
+
+            await asyncio.gather(*probe_tasks)
+        finally:
+            #  We may have been cancelled, so we should cancel
+            #  the probe tasks we started to clean up
+            for task in probe_tasks:
+                task.cancel()
+
+
+#  Wait until a SPACE character to be read on stdin.
+#  Afterward, cancel the probe task so we can exit
+# async def wait_for_spacebar(probe_task):
+#     exit_event = asyncio.Event()
+
+#     def read_callback():
+#         #  Read a single character
+#         #  If we tried to read more, we may block other tasks
+#         key = sys.stdin.read(1)
+#         if key == ' ':
+#             exit_event.set()
+
+#     loop = asyncio.get_event_loop()
+#     loop.add_reader(sys.stdin, read_callback)
+#     await exit_event.wait()
+#     loop.remove_reader(sys.stdin)
+
+#     #  After spacebar is pressed, stop sending probes
+#     probe_task.cancel()
+
+
+#  The main asynchronous routine, running within the asyncio event loop
+async def main_task(hostname):
+    screen = curses.initscr()
+    try:
+        probe_task = asyncio.ensure_future(
+            launch_probes(screen, hostname)
+        )
+        # spacebar_task = asyncio.ensure_future(wait_for_spacebar(probe_task))
+
+        try:
+            # await probe_task
+            await asyncio.gather(probe_task)
+        except asyncio.CancelledError:
+            #  It is normal for probe_task to be cancelled by
+            #  the spacebar task
+            pass
+        finally:
+            #  We need to clean up by cancelling if gather has returned
+            #  early, perhaps due to an exception raised in one of
+            #  our tasks.
+            probe_task.cancel()
+            # spacebar_task.cancel()
+    finally:
+        curses.endwin()
+
+
+#  Get the hostname to trace to on the commandline
+if __name__ == '__main__':
+
+    host: str = '192.168.1.68'
+    if not len(sys.argv) > 1:
+        print('Usage: python3 trace-concurrent.py <hostname>')
+        sys.exit(1)
+    else:
+        host: str = sys.argv[1]
+
+    out = asyncio.run(main_task(host))