Implementing periodic boundaries for rigid bodies

Lots of the changes here are entirely cosmetic or else were intended entirely for my use - e.g. most of the new comments and the changes to pymolwrapper.py which were designed to allow visualisation of small rigid-body periodic systems.

The important changes are the ones where I've defined new or overloaded functions and classes. In particular the new _otp_bulk module, the new classes in minpermdist_stochastic.py and periodic_exact_match.py, and the changes to the cpp files/their wrappers. I've also put in a fairly extensive test routine.
True, this is a bug, it should be

    for p in ca.rotRigid:
        p[:] = rotations.random_aa()

This is already fixed in the current version of master
https://github.com/pele-python/pele/blob/master/pele/angleaxis/aasystem.py#L59