add ultranest and snowline samplers

[JohannesBuchner]

This is a work-in-progress to add two more samplers:

1. https://johannesbuchner.github.io/UltraNest/ is a nested sampler that can be operated in two modes: (1) with MLFriends, a parameter-free safe algorithm, efficient in low dimensions and (2) with slice-sampling.
2. https://johannesbuchner.github.io/snowline/ combines a optimizer (iminuit) with importance sampling.

The commits provide some basic documentation, and allows sampling.

Reading the output posteriors and evidence still needs work.

Please let me know if you are interested in merging this in principle.