[WIP] Fix PPVCube

[jzuhone]

The PPVCube class is out of date with both yt and yt_astro_analysis. This PR brings it up to date, in particular:

• Removes deprecated keyword argument depth_res, which was originally here for off-axis projections and is now long gone in yt.
• Does not assume that the field type will always be "gas".
• Uses sampling_type="local" in fields to allow for SPH projections.
• Makes sure field tuples are always used.
• Removes code used to create line-of-sight velocity fields and uses the built-in yt functionality for this purpose.

Still need to fix the docs, hence WIP. This appears to have uncovered a bug in yt which I'll have to resolve separately.

[brittonsmith]

@jzuhone, I've looked this over and it all looks reasonable to me. It sounds like we may try to do a new yt_astro release soon. Should we wait for you to finish the docs so we can include this?

[neutrinoceros]

To be fully transparent there are no real blockers if we want to do a release sooner rather than later. The release process is also smooth enough that we can easily cut one when this goes in, regardless of exactly when it happens :)

[spectram]

@jzuhone Hi, your ppv branch looks great. I am interested in implementing these changes in my branch as well. I am keen to attempt a rebase. Please let me know if that would be helpful to address the conflicts and close this PR.