sets the facets for a TetGen expression.
- To use TetGenSetFacets, you first need to load it using Needs["TetGenLink`"].
- Each facet is a list of indices into the point specification.
- Each facet represents a polygonal region; it may have any number of sides and may be nonconvex, possibly with holes, segments, and vertices in it.
- Each facet must be coplanar.