Delaunay triangulation based "hull" and hull_volume

This mainly adds a couple of modules (hull and hull_volume) which will be useful if we explore interfaces based on Delaunay triangulations, which I think might be particularly nice to do if the MSM cluster centers are clustered in a reduced dimensional space. I’m keeping hull.py separate from hull_volume.py because hull.py is duplicated in my own code. (Issuing the pull request now to keep it distinct from the XYZ stuff I’m still working on.)