Finished implementing the mixture_density class.

Todo: Choose a method of tying state emissions. Currently this is
implemented at the state level, ie overall emission density functions
are tied. Two possible methods are available:

(1) Tied-Mixture Distributions --> mixture densities are state
independent. All state emissions have different linear combinations of
the same set of mixture densities.

(2) Sub-State Tied Mixtures --> Certain sets of mixture densities are
tied together. here the emissions per state are viewed as a mixture of
sub-mixtures.