Make ray an optional dependency

[mdbenito]

Description

This PR closes #380 #367 .

Changes

Because we are already using a sub-class registry, there was little to do:

• Moved ParallelBackends into separate modules
• Guard imports
• Adapt the tests to skip if ray is not available. Not very elegant, but the alternative was to change the fixture parallel_backend and that would have meant a lot more changes. Ideas? (I'd just leave it as is tbh)
• Documentation is missing, waiting until we have Ditch sphinx #261 to avoid silly conflicts

Checklist

• Wrote Unit tests (if necessary)
• Updated Documentation (if necessary)
• Updated Changelog
• If notebooks were added/changed, added boilerplate cells are tagged with "nbsphinx":"hidden"