Improve how sampling seed is handled

This MR aims to standardize how the sampling seed is handled for the different samplers. This is motivated by bilby_pipe#240 (closed).

Proposed change

Check if a sampler supports setting a random seed (sampling_seed_key) and, if so, translate the seed to the correct keyword argument. Rather than add the "translation" to each sampler individually I've opted to change _translate_kwargs in the base sampler class and have each child class call the parent method.

bilby_pipe

In bilby_pipe we can check if a sampler supports a sampling seed (sampling_seed_key is not None) and add the correct key if one of the equivalent keys isn't already set.

To-do

• Check if documentation needs updating
• Test alongside changes to bilby_pipe to fix bilby_pipe#240 (closed)