UltraNest support

[JohannesBuchner]

Would you be interested to add support for https://johannesbuchner.github.io/UltraNest/ ?
The interface should be very similar to pymultinest.

UltraNest is a very reliable tuning-parameter-free algorithm. I have published some examples where the UltraNest algorithm is unbiased while pymultinest gives a different answer. UltraNest is a pure-python package and very easy to install with pip or conda.

UltraNest also supports resuming from disk and MPI parallelisation, if that is useful to you.

[ideasrule]

I'll definitely check it out! Based on your documentation, it seems better than dynesty in both speed and performance. If that's the case, I might as well replace dynesty with ultranest.