Delaunay refinement (#66)

[Stoeoef]

See #66.

This is the initial implementation which works sufficiently well already.

To use this, create a CDT, insert some constraint edges and then call

my_cdt.refine(RefinementParameters::new().exclude_outer_faces())

which will refine the cdt inplace. Note that, in order for exclude_outer_faces to work, the constraint edges should form a loop somewhere - otherwise, all faces will be considered to be outer faces and won't be refined.

Remaining TODOs (just mentioning for my own overview):

• [x] Documentation of AngleLimit
• [x] Documentation of RefinementParameters
• [x] Documentation of fn refine
• [x] Internal documentation by adding a few comments at appropriate places
• [ ] At least some improved test coverage, e.g. for testing special cases
• [ ] Expand fuzz tests (those are more or less just stubs at the moment)

TODOs for some follow-up PRs:

• Try to enable refinement algorithm for regular triangulations (if easy)
• Investigate if the refinement quality can be increased (though it's not too bad already)
• Add some images detailing how exclude_outer_faces works