Create a double-to-single backtick migrator

[larsoner]

In MNE-Python (and it sounds like elsewhere), one way of working around single-backtick parameter names not cross-linking and emitting nitpicky warnings etc. has been to use double-backticks. We should write a little utility to go through docstrings, find parameters, look for double-backtick-enclosed versions of those, and replace with single backticks.

I can work on this at some point perhaps! I have code to test it on 🙂

[bsipocz]

astropy and related projects also double backticked all parameter names, I can offer astroquery and pyvo for testing ground if we need some smaller projects.
Though any such PRs will touch a lot of files/lines, so a tool that does it incrementally, just on the diffed files would be nice, too.