Enforce edges

[jdugge]

Some edges such as shorelines or ridges should be manually enforced in the triangulation, instead of relying on the algorithm to find them.

Implement the algorithm from Shewchuk, J.R. and Brown, B.C., 2015. Fast segment insertion and incremental construction of constrained Delaunay triangulations. Computational Geometry, 48(8), pp.554-574.