UltraNest support

[JohannesBuchner]

Would you be interested to add support for https://johannesbuchner.github.io/UltraNest/ ?
The interface should be very similar to dynesty.
UltraNest is a very reliable tuning-parameter-free algorithm. It can be faster than dynesty, especially if you take advantage of vectorization.
It also supports resuming from disk and MPI parallelisation, if that is useful to you.

[bd-j]

Hi @JohannesBuchner, yes that would be great. If the API is close to dynesty it shouldn't be too difficult, the main tasks being a wrapper to run the fit (in prospect.fitting.nested), a task to make the output -- samples, weights, etc. -- writable in the standard format (in prospect.io.write_results), and adding command line arguments to support UltraNest options.

I don't have time to put this together at the moment, but would be happy to review a PR.