add find-closest-point method

[kaveh808]

Implement a method find-closest-point for the class polyhedron. Ignore the shape's transform.

The returned point must be on the surface of a face, on an edge, or at a vertex or the polyhedron.

[kaveh808]

@foretspaisibles Would you be interested in tackling this issue? You seem to be mathematically inclined. :)

[foretspaisibles]

@kaveh808 Ah ah yes, I actually paid some thoughts to it and try to figure out how to find the closest points on two disjoint polyhedrons. Need to experiment a bit with this!

[kaveh808]

@foretspaisibles Cool. I am actually looking for a point to a polyhedron, not two polyhedra. If necessary, you can make the simplifying assumption that the polyhedron only has convex faces, or even is triangulated.