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.
> # sn402: does this mean call the basesystem get_takestep(), or call it on _otp_bulk and look for the first inherited one?

`self` refers to an instance of OTPBulk, so it would call OTPBulk.get_takestep(), which probably would actually be implemented in AASystem.get_takestep().