Merging months of changes into master

See merge request daniel.wysocki/bayesian-parametric-population-models!13

# Conflicts:
#   requirements-docs.txt

To get old behavior use `beta_q = 0.0`

Joint PDF function wasn't working unless `where` was a proper numpy
array, as it used the `.T` attribute to transpose.  Fixed this by
promoting `where` to an array.

Factored out the prior parsing code into a function, so that it can be
used by external programs.

Now making heavy use of ``where`` and ``out`` arguments even within
the specific models.  Also reverted to the split before/after prior
aux functions, rather than the single unified ``aux_fn``, with the
added ``prior_support`` argument.  This allows efficient use of the
``where`` argument.