Clean up dynesty

This attempts to clean up the dynesty implementation. With lots of different options it was getting a bit spaghetti like.

Note, this removes the 'dynamic' option. We can add this back in later as a separate sampler.

mentioned in issue #151 (closed)

Great, I think I was responsible for most of the ugliness here, thanks for cleaning it up.

merged

mentioned in commit 0dd2f4cd