teach lowranceusr about track segments

[tsteven4]

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.

This resolves #1305

[codacy-production[bot]]

Coverage summary from Codacy

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Coverage variation	Diff coverage
:white_check_mark: +0.18%	:white_check_mark: 64.71%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (c86aff1d7068edf9a987860d88ebe02a56674157) | 27636 | 17211 | 62.28% | | | Head commit (00ff2f674baf1ee863cff3ee5d0ba7a26d4549f8) | 55289 (+27653) | 34531 (+17320) | 62.46% (**+0.18%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#1306) | 17 | 11 | **64.71%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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