Rename fine distributions to BMMs

[pawel-czyz]

@grfrederic This is an ongoing effort to replace fine distributions with BMMs (as described in #143). This requires considerable changes:

• Update the imports in src/ (and make sure that the tests/ pass as they will also have to be updated.)
• Update the names, such as FineSampler to BMMSampler.
• Update the snakemake workflows (this is very important!)
• Revise the documentation on fine distributions. Check if API is rendered properly.

Advice, comments, or changes pushed to this branch are welcome 🙂