Get rid of p_o_i and p_o functions

[franknoe]

I have noticed that I am not using the output_model functions:

bhmm.output_models.discrete.p_o_i
bhmm.output_models.discrete.log_p_o_i
bhmm.output_models.discrete.p_o
bhmm.output_models.discrete.log_p_o
bhmm.output_models.gaussian.p_o_i
bhmm.output_models.gaussian.log_p_o_i
bhmm.output_models.gaussian.p_o
bhmm.output_models.gaussian.log_p_o

In favor of a smaller interface I would like to remove them unless you believe we need them.

[jchodera]

Sounds good! Happy to submit a PR for this once we have fixed the failing nosetests.

[franknoe]

I have commented out these functions for now. If we're sure we don't need them, we can remove the code.