Contour Polygons with Marching Cubes

Previously, the Marching Cubes contouring algorithm only had case tables for 3D polyhedra. This means that if you attempted to contour or slice surfaces or lines, you would not get any output. However, there are many valid use cases for contouring this type of data.

This change adds case tables for triangles and quadrilaterals, and lines. It also adds some special cases for general polygons and lines. These special cases do not use tables. Rather, there is a special routine that iterates over the points since these cells types can have any number of points.