Detection of intersecting segments in user input

[DanielVandH]

Currently, the method for constrained triangulation assumes that segments do not intersect, although collinear segments are automatically detected (most of the time, floating point arithmetic can only do so much). It would be nice to have a way to automatically detect this in a user's provided edge input. e.g. some routine like

1. Merge the provided segments and boundary edges into a single set (merge_constrained_edges does this).
2. Compute all intersections between the segments, recording for each intersection a tuple (p, segments), where p is the intersection point and segments are the segments involved in the intersection.
3. For each (p, segments), where segments is say (c1, c2, ...), split each ci in segments at p. A method like split_constrained_edge can do this, similar to how we handle it in process_collinear_segments!. Make sure boundary edges appropriately handled.
4. Proceed with the triangulation.

Maybe add_edge! could have an initial seeking step that looks through all inserted segments (all_constrained_edges) and just finds intersections there, in case users add edges after the initial construction of a triangulation. An option could be toggled to allow this, so that we disable it while the triangulation is being built but when using add_edge! after the fact it is enabled by default.

[DanielVandH]

Reopening. Maybe this can't go in the package directly, or it can and we just need to document examples of it properly and allow it to be done outside of triangulate as well so that users can keep the results for example.