Node_em_gmm and mixture objects

Currently Tied emissions are at the state level ie the overall
observation density function for the state.

This needs to be further partitioned to the individual mixture
densities per state.

This will be implemented via extending the node_em_gmm object to have a
collection of gaussian mixture objects.

Each gaussian mixture object will have a mean, covariance, precision,
and name. (mixture density object?)

I will need to update the HMM, State, node_em_gmm classes and the
learning algorithms. The various interfaces should not need to be
changed (too much as only the mixture id will need to be additionally
passed).