Merge/fxadv

[oelbert]

Purpose

This PR merges stencils in fxadv. Hopefully it will pass tests soon.

Code changes:

• fvadv.py: Turned most stencils into gtscript functions and created fxadv_stencil and fxadv_corners_stencil that call these functions, reducing stencil call counts.

Checklist

Before submitting this PR, please make sure:

• You have followed the coding standards guidelines established at Code Review Checklist.
• Docstrings and type hints are added to new and updated routines, as appropriate
• All relevant documentation has been updated or added (e.g. README, CONTRIBUTING docs)
• Unit tests are added or updated for non-stencil code changes

[rheacangeo]

Note the regions/fxadv branch is the single stencil version. I closed the PR for it because it didn't seem regions were going to change anytime soon. That branch is out of date with current master, however. When we have regions for gt backends, we should attempt to make it all a single stencil.