ENH: add API to switch raytracing engine at runtime

[neutrinoceros]

PR Summary

This allows to properly define pytest fixtures for switching between "yt" and "embree" ray tracing engine options at runtime (useful in #4122)

Also, a follow up to #3626

[matthewturk]

Nice!

[neutrinoceros]

@matthewturk I just realised I don't need the boolean flag, give me a minute to remove it and then you can set this to auto-merge if you'd like

[neutrinoceros]

Sorry about the additional iterations, I think it's good now.

[neutrinoceros]

sigh... actually I do need it, otherwise my fixtures just crash pytest

[neutrinoceros]

Nevermind. This is usable as is, and validated via #4122

[neutrinoceros]

A problem I failed to see originally here is that "embree" is currently the default ray tracing engine. In the current state, this means yt will raise a warning at startup time unless one of the following conditions is met:

• embree is correctly installed
• the parameter is override from the config file

This isn't acceptable.
So here are the options I can think of:

1. Change the default engine to "yt". This has the downside of breaking users relying on being able to use embree with no additional configuration (other than installing it).
2. reintroduce a very boolean flag so that we can filter the warning, and use it specifically at initialisation (adds complexity to an already very complex code section)
3. never warn the set_raytracing_engine("embree") fails

None of these are ideal. I think 1) is the correct thing to do, provided the breaking change clearly mentioned in release notes. Notes on 1):

• it doesn't force users to start using configuration files since they also get the option of switching engine at runtime.
• pyembree sadly seems to be kinda discontinued. It's never been trivial to install without conda and even there, conda-forge hasn't received any new binaries for it in 2.5 yrs at this point, which means we can only test it simply on conda + Python 3.8. See https://anaconda.org/conda-forge/pyembree/files

Hopefully this feels acceptable not just to me, I'll let reviewers discuss this point.

[neutrinoceros]

I've now implemented my preferred solution (1)