DRAFT: Mh-mclmc

[reubenharry]

This is a draft of MH-MCHMC. Next step is to incorporate the recent updates to the proposal code.

Checklist:

• If I add a new sampler, there is an issue discussing it already;
• We should be able to understand what the PR does from its title only;
• There is a high-level description of the changes;
• There are links to all the relevant issues, discussions and PRs;
• The branch is rebased on the latest main commit;
• Commit messages follow these guidelines;
• The code respects the current naming conventions;
• Docstrings follow the numpy style guide
• pre-commit is installed and configured on your machine, and you ran it before opening the PR;
• There are tests covering the changes;
• The doc is up-to-date;
• If I add a new sampler* I added/updated related examples