[WIP] xy-grid extrusion along z

[matthewturk]

This implements the first steps toward a data index that is defined by an xy grid of points with constant z. For instance, the particular use case is ARM data that is taken at multiple sweeps along the azimuth, at roughly onstant elevation, and always constant "range" (distance). This could be defined as a hexahedral mesh, but because we have a reduced dimensionality in xy, it can reduce memory considerably to define it with xy_grid and z_grid.

Also! I may have messed up my git branching! So there's the likely possibility that I'm including irrelevant changesets! But, when Nathan's PR goes in and I rebase, maybe that will go away?

There are numerous todo's, including:

• finish implementation of extrusion base data object
• finish fwidth
• integrate into pixelizers the new extruded mesh pixelizer
• implement cf_sweep frontend
• Code passes flake8 checker
• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

[matthewturk]

I'm converting this to draft until after 4.0 goes out so that it won't show up in my filters. This is a worthwhile change, but I need to update it and move forward with an updated version.

[matthewturk]

I do want to revisit this, albeit perhaps with a rebase, at some point in the future.