Release 2.1.6 Iterators, Contouring, and Natural Neighbors

Release 2.1.6 fills out the set of iterators for the incremental TIN classes to include the fundamental elements of a Delaunay mesh: edges, triangles, and vertices. It includes substantial revisions and improvements to the contouring classes. It provides bug fixes for the Voronoi rendering operations. And, finally, it cleans up some API elements for the Natural Neighbor Interpolator.

For 2.1.6, we've also started maintaining a new documentation page to provide better subject-matter organization and search capabilities. Additionally, the documentation now includes a description of Tinfour's unique algorithm for Natural Neighbor Interpolation (see https://gwlucastrig.github.io/TinfourDocs/).