ENH: add npool_post_process

[mj-will]

Add an npool_post_process.

This allows the user to configure how many processes are used for post-processing independently of how many are used for sampling. Whilst making this change, I also changed the code so aliases for npool also trigger the parallelized post-processing.

I also remove an old unused argument.

[ColmTalbot]

With this method, is the way to disable the parallelisation fo this stage to pass npool_post_process = 1?

[mj-will]

With this method, is the way to disable the parallelisation fo this stage to pass npool_post_process = 1?

Hmm, good point. By default npool=1 still creates a pool in other parts of bilby so that would be inconsistent. I'll have a think about how to best handle this.