Refactor samplers

Closes https://git.ligo.org/Monash/tupak/issues/163

This MR moves sampler.py to a new directory and each sampler gets its own file. There should be no changes to the U.I., although if people can think of good tests of this to do that would be great.

This branch was created from https://git.ligo.org/Monash/tupak/merge_requests/183 in order to keep the new docstrings.

@gregory.ashton @moritz.huebner