c++ angle axis (rigid body) framework

this is a partial c++ rigid body framework, just enough so that potential calls are entirely in c++.

The only difference from a python users's perspective is you use a different implementation of RBPotentialWrapper.

I implemented a hacky version of a matrix class.  We should think about ways to do this better, or use an existing external implementation.
change this to reference, does have performance impact (at least in VS2013)
inline dtype const &operator()(size_t i, size_t j) const