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.

- If no saved data exists when loading, a valueerror is now raises. This
  catched that exception to work as expected.

- Adds a filename option for direct loading
- Raise an error if no information given

Moritz Huebner: Made sampler.result a property and moved some of the stuff that happens in run_sampler in there.

If DISPLAY environ not found, then default to the Agg matplotlib
backend. Useful for running on clusters etc.

Moritz Huebner: reintroduced kwargs property/setter, is probably better because that behaviour is getting inherited