Previously, `parameters` in `fill_prior` was only ever used to fill in
the non-standard parameters if sampling in something other than the
defaults. This directy checks that. The advantage is a cleaner logic and
we no longer assume the likelihood has a
`non_standard_sampling_parameter_keys` attribute.

This reverts commit 3860e42d. Which I
mistakenly removed.

Add uniform in log prior

See merge request Monash/tupak!42

Change sampled parameters

See merge request Monash/tupak!31

# Conflicts:
#   tupak/sampler.py