support general polyhedra from triangular meshes

It would be nice to support importing a general triangular mesh for an object, e.g. from gmsh or an STL file, in order to define an arbitrary tetrahedral surface.

There are some functions in SCUFF for this kind of thing that we could borrow, e.g. https://github.com/HomerReid/scuff-em/blob/9c6d0cb7695463af803dee8d04cdae939740cdcc/libs/libscuff/PointInObject.cc