Overhaul facial adjacency + boundary tagging

[majosm]

This PR:

1. Restructures Mesh's facial adjacency structure to allow multiple adjacency groups for each pair of mesh element groups. In other words, List[Dict[FacialAdjacencyGroup]] becomes List[List[FacialAdjacencyGroup]].
2. Splits FacialAdjacencyGroup into InteriorAdjacencyGroup and BoundaryAdjacencyGroup subclasses (with InterPartitionAdjacencyGroup inheriting from BoundaryAdjacencyGroup).
3. Splits boundary adjacency for different boundary tags into different groups.
4. Splits inter-partition adjacency for different ranks into different groups.
5. Removes boundary_tags/boundary_tag_bit from Mesh.
6. Adds affine transformations to InteriorAdjacencyGroup/InterPartitionAdjacencyGroup.

This will almost certainly need to be split into multiple PRs, but I wanted to throw this up here first for CI + discussion.

(Probably incomprehensible unless read commit-by-commit.)

[majosm]

(Looks like I need to do a little more work on the BTAG_NONE/BTAG_ALL/BTAG_REALLY_ALL handling. Stay tuned. 🙂)

[majosm]

Just looking for a review of the high-level overview in the OP to check if this seems reasonable (though feel free to take a glance through the code if you're feeling particularly adventurous. 🙂) After that I'll split this up into smaller pieces.

[inducer]

Thanks for working on this! Here's an initial review up to (but not including) 5cda667 ("remove unnecessary intermediate steps in partition_mesh")

[majosm]

I think this is ready for another look @inducer.

[thomasgibson]

Some comments/touch-ups in places after reading through the changes. Thank you for spending the time doing all this!

[inducer]

Thanks for working on this, and thanks @thomasgibson for taking a look!