Extending support to grids with arbitrary `face_connections`

[hdrake]

Example: MITgcm's LLC grids.

What would be needed:

Phase 1: only operations involving volume-integrals (i.e. ignoring along-boundary structure of transports)

• Pass face connections to xgcm.Grid constructors, e.g. https://github.com/hdrake/xwmt/blob/03b405bfd00faa9536af193ad53abaeb9d4c5224/xwmt/wm.py#L49-L59
• Make the xbudget dictionary, following the MOM6 example: https://github.com/hdrake/xbudget/blob/main/xbudget/conventions/MOM6.yaml
• Extend grid operations (e.g. horizontal sums and integrals) to broadcast across faces as well, e.g. here: https://github.com/hdrake/xwmt/blob/03b405bfd00faa9536af193ad53abaeb9d4c5224/xwmt/wmt.py#L559-L563
• Add warnings that grids with face_connections can only use volume-integral methods

Phase 2: include support for along-boundary structure of transports (i.e. generalizing regionate and sectionate

[hdrake]

@liu-ran and @anthony-meza, here are some thoughts on a minimal work plan for extending xwmb support for MITgcm/ECCO