[IMP-dev] too many rotations in IMP

I think we currently have too many way of representing matrices in the IMP API. - Rotation3D - EulerAnglesZYZ - and EulerMatrix

The last doesn't make any sense as there is no reason to tie together two disparate representations. Given that one can create a Rotation3D from Euler angles I don't see any reason to expose the second. If we find rotational matrices are faster than quaternions for actually performing the rotations, then we should cache the matrix in Rotation3D rather than add another class.

Comments? Justifications for having many different versions?