Do not discard minimum-length paths in FlexPath::remove_overlapping_points()

[jatoben]

Currently, FlexPath::remove_overlapping_points() removes points from a path if the length is less than or equal to the path's tolerance. But the equality test discards a minimum-length path, which is exactly one database unit in length.

I believe this is what's occurred in #268 and #277, both of which contain paths that have a length of 0.001 microns. That's the value for path tolerance when the library has the default unit and precision values of 1e-6 and 1e-9, respectively.

This PR updates remove_overlapping_points() to discard points only if the computed length is strictly less than the tolerance, and adds a couple of tests.

[jatoben]

Separately, I have another local patch which adds an EmptyPath error code value, and returns it from FlexPath's ::to_gds(), ::to_oas(), and ::to_polygons() methods when an empty path is detected (the ::to_polygons() change is needed to catch the non-simple path case). The Python bindings raise a warning if this error code is returned, allowing callers to detect a path of less-than-minimum length when writing out a library.

I'm not quite sure about this change though, since FlexPath seems to intentionally return no error when the path has too few points.

gdstk/src/flexpath.cpp

Lines 916 to 921 in 18167ef

	ErrorCode FlexPath::to_oas(OasisStream& out, OasisState& state) {
	ErrorCode error_code = ErrorCode::NoError;
	remove_overlapping_points();
	if (spine.point_array.count < 2) return error_code;

gdstk/src/flexpath.cpp

Lines 240 to 242 in 18167ef

	ErrorCode FlexPath::to_polygons(bool filter, Tag tag, Array<Polygon*>& result) {
	remove_overlapping_points();
	if (spine.point_array.count < 2) return ErrorCode::NoError;

@heitzmann, let me know what you think about the error handling here -- I'm happy to send in the change as a follow-up PR if you'd like.

[heitzmann]

@jatoben Thanks for looking into the issue!

Yes, your changes to add EmptyPath error make sense to me. Feel free to open a PR for them!