Floating base

Adds support for quaternion floating base parameterization and having dq/dt and v be different. 

Not ready to merge. Still need to do mex implementation. I believe that at this point, all the tests that I can get to pass without mex do pass.

Comments very welcome.
Sorry, I was confused when I read the documentation. In the first line of the documentation it says 'in base frame'. I think 'body_or_frame_ind' is a good name choice, as we can have body_ind or frame_ind. But if we use 'base_or_frame_ind', does it mean we can have base_ind or frame_ind?