Fixing sweep durations that cause weird tracker behaviour

The tracker plugin requires that the duration of each sweep be accurately known, otherwise it can generate
spurious tracks and misconnect new blips to existing tracks.  This problem sometimes appears in blipmovies
generated by converting RTI .REC files (the source is the issue is under investigation).

The highlighted track in the screenshot below is an example of such a misconnection:

is an

On a typical radar spinning at 25 rpm, the sweep duration is 2.4 seconds.  In blipmovies, this value is available from the R console like so:

RSS$scan.info$duration

The value is in milliseconds, so for a typical radar, the value should be around 2400.  The value for the
sweep shown above was 214798 milliseconds, which is obviously very wrong.

If you see strange behaviour in the tracker, especially sudden misconnections among existing tracks,
or long new segments attached to previously reasonable tracks, your blipmovie might have (occasional)
sweep duration errors.  If you suspect this, try paste the following into your radR console window, then
run the tracker on the suspect portion of a blipmovie.  If the results in the console window include values
very different from 2400, then you should correct the blipmovie (see below).

rss.add.hook("DONE_SCAN", function(...) gui.put.cons(paste("Sweep at", capture.output(round(RSS$scan.info$timestamp)), "is", round(RSS$scan.info$duration), "milliseconds\n")))

Here's the result from a blipmovie segment with bad durations (one of which is highlighted):