Initial (cleaned) implementation of VBMFMM in 3D.

Implements variational inference for the 3D von-Mises Fisher mixture
model with conjugate priors on the cluster means, precisions, and
weights.
Added basic tests for the 3D VBMFMM.
Added example for VBMFMM in 3D.
Generalized model to other dimensions than 3.

Did not generalize tests since I know of no way to sample from the vMF
distribution in dimensions other than 3.