Mixture_density class implemented

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

Two versions of tied mixtures:
(1) One set of mixture densities for all of the states (state
independent) and the state emissions are different weighted sums of
these mixture densities. So each state emission is a different weight
vector.
(2) There is a sub-state tying of mixture distributions. This will
involve using different update formula which is optimal in its
optimization and tying.