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.
Yeah you can use either a rigid body index or a frame index, so in that sense it's symmetric now. In fact, `bodyKin` now just calls forwardKin with `body_or_frame_ind` and `base_or_frame_ind` swapped. I just rewrote the documentation to make it (hopefully abundantly) clear what the arguments and return values are.