GpxLongitude handles +180 incorrectly

[airbreather]

In the GPX schema, longitudeType's upper-bound is 180, exclusive.

GpxLongitude, however, allows values with +180.

This mismatch makes it possible, under normal circumstances, to produce values that are invalid for GPX.

Legal options:

1. Keep allowing +180 in GpxLongitude, and convert it to -180 only when writing XML
2. Reject +180 in the GpxLongitude constructor; convert +180 to -180 when converting from NTS
3. Treat +180 and -180 as the same value
4. Convert +180 to -180, silently, in the GpxLongitude constructor

[airbreather]

Keep allowing +180 in GpxLongitude, and convert it to -180 only when writing XML

This feels like the way to go. Perhaps a GpxWriterSettings flag could be added that allows technically violating the schema and skipping this step.

Hmm, on second thought, not right now.

Reject +180 in the GpxLongitude constructor; convert +180 to -180 when converting from NTS

This is actually slightly better. If we start out by making something an error, then we can relax that error condition later without worrying about breaking working code.