This reverts commit 46b9e61a.

Note - also forces all `__init__` calls in `prior.py` to use key word
arguments to ease changes in the future.

* Adds a CLI arguments `sampler-help` which prints the docstring for the
  given sampler
* Documents each Sampler class with details of the commonly-used kwargs
  and information on where the kwargs are passed (except for pymc3)

This reverts merge request !171

Previously variables with a fixed (or delta function) prior where not
added to the posterior data frame; the documentation for
`samples_to_posterior` indicates that this is the case. This resolves that.

Fix 174