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

prior.py: do not set a dist variable containing the scipy distribution, just call the distribution functions as needed

 - rework how the Pymc3 sampler class sets the priors

 - move the setting of the PyMC3 priors into the Pymc3 sampler
   class

 - this is the start of adding functionality to use the PyMC3
   sampler. There is a lot more that needs doing, but the basics
   work.

By putting in the dictionary as an ordered dict, the way priors are
entered determines the order they are plotted, which is what most people
will expect.

- Also adds some catches to help improve error messages for when the
  input prior is something else.

Resolve "`sample_subset` in `PriorSet` should not have have `list()` as a default argument for `keys`"

Previously, outdir was checked and if-not-existing created in a few
different places in two different ways. This (a) makes them all use the
same method (namely the utils function) and (b) places a
check-and-creation call before every tupak output. This will ensure that
changes elsewhere in the code won't cause bugs in the future. Also, if
you choose not to save anything, it won't just make an empty directory.