Fix broadcasting in calculating emission log probs

[gileshd]

Fix reshaping of emissions so that broadcasting works with HMM.emission_distribution.log_prob() works for both scalar and vector observations.

The behaviour is now:

• scalar - (T,) --[reshape]--> (T,1) --[tfp broadcast]--> (T,K).
• vector - (T,D) --[reshape]--> (T,1,D) --[tfp broadcast]--> (T,K).

This fixes many methods in HMM subclasses with scalar observations (e.g. CategoricalHMM.filter(), CategoricalHMM.smoother(), etc.).

The PR also changes a line in BaseHMM.log_prob() to only sum over the leading axis, this helps for a failure case whereby extra dimensions in the input can cause broadcasting of the tfp log prob which is subsequently hidden by a .sum().

With the present change the method will output an array with shape determined by the broadcasting (e.g. if x is shape (T,) inputing x[:,None] will now output an array of shape (T,) rather than () which at least makes it clear that broadcasting has occurred). Going forward it might be useful to have some shape checking steps.