Nested sampling with MultiNest

[tomasstolker]

This PR adds the MultiNest class to the sampler module. MultiNest supports the use of MPI, so it can be executed as e.g. mpirun -n 100 python run_orbitize.py if 100 CPUs are available.

Also made some minor modifications in NestedSampler that uses Dynesty. I added the nlive parameter, which is used by the static sampler. I removed a ValueError, which seemed not needed since pfrac is not used when static=True.

The unit tests do not actually call run_sampler, since MultiNest needs to be manually installed.

Let me know if you have any comments/feedback!