Implement nautilus sampler interface

[johannesulf]

This PR adds the nautilus sampler (https://nautilus-sampler.readthedocs.io/). I could need some help ensuring that everything works as expected.

• blobs: Nautilus supports blobs. The interface is very similar to emcee's. But I still need to figure out how to give those results back to CosmoSIS.
• MPI: Nautilus supports MPI parallelization via schwimmbad or any pool with a map method, just like emcee and dynesty. Seeing how dynesty and emcee are interfaced, MPI parallelization may already work. But that needs to be tested.
• tests: No unit tests have been implemented yet.

[joezuntz]

Hi @johannesulf - thanks so much, this is brilliant! I'll look at the interface for derived parameters this week.

[joezuntz]

@johannesulf - I've made some changes - thanks for giving me access.

I've added the blob thing, that worked fine thanks!

For the "resume" feature I had to add a few bits of supporting code to the wider cosmosis, which is really useful elsewhere too. I've also added the postprocessing and a quick test.

[johannesulf]

Wonderful! I quickly tested MPI and it seems to work fine. The implementation of blobs looks very good. But I haven't done any extensive tests.

[joezuntz]

Thanks so much for this - looking forward to testing it out.