Refactor walk_volume in volume rendering

[matthewturk]

PR Summary

In order to support non-Cartesian volume rendering, we need to be able to switch the volume walk algorithm on the fly. This sets up the preliminary means to do that, by turning walk_volume into a function pointer, and (similar to the smoothing kernel work) using a getter function that looks up the type of geometry and supplies the correct one.

PR Checklist

• pass black --check yt/
• pass isort . --check --diff
• pass flake8 yt/
• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

[cphyc]

Note that this PR likely clashes with #2610, so it would be worth joining efforts!

[matthewturk]

@cphyc !!! you're right, how did I miss that?!

[cphyc]

So after a more thorough inspection, it seems these two PRs aren't clashing that much, but it would be great to disentangle the code so that we have

• a cartesian ray caster on grid-based datasets
• a cylindrical ray caster on grid-based datasets (this PR)
• a cartesian ray caster on oct-based datasets (Volume rendering for octrees #2610)

organized in some clever way. I'm open to suggestions to work on #2610!

[matthewturk]

I'm converting this to draft until after 4.0 goes out so that it won't show up in my filters. It also needs to be updated with @cphyc 's suggestions and changes.