teach lowranceusr about track segments (#1306)

teach lowranceusr about track segments (#1306)

* teach lowranceusr about track segments.

lowranceusr versions 2 & 3 have a continuous flag used to
indicate dicontinuities in a trail.
The lowranceusr version 2 & 3 reader now flags trail discontinuities
as new track segements.
The lowranceusr version 2 & 3 writer now flags new track segments as
discontinuities.

Also fix two subtle bugs:
1) when reading a version 2 or 3 trail with the break option we would
fail to break the trail if the discontinuity was at the start of a
section other than the first section.
2) when writing a version 2 or 3 trail with the merge option if the
number of points exceeded MAX_TRAIL_POINTS we would write a header
that indicated MAX_TRAIL_POINTS but actually write MAX_TRAIL_POINTS + 1.

* add lowranceusr test for track segments.