LLC90 Heat Salt and Mass budgets on native coordinates

[anthony-meza]

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[anthony-meza]

Brief summary @hdrake: I've added ECCO V4r4 support to xbudget!

This required me to add the following source code:
llc90: A folder that contains code to calculate horizontal differences on the LLC90 grid (operations.py) as well as code to process the standard ECCO output from PO.DAAC (budget_diagnostics.py).

collect.py: Added a lateral_divergence operation that currently only calls operations.py since it is currently only needed to operate on the tricky LLC90 grid. I have not written a test for this operation because it only works with the LLC90 grid output. In the future, this particular operation could just use an xgcm operation, but currently, I'm not able to figure out what the problem is.
PS: lateral_divergence could be extended to support other non-LLC90 grids, but it would require repeating a lot of the code found in difference, so for now, I have not added it.
PSS: lateral_divergence is maybe a bit of a misnomer, since it's really only taking differences and not dividing by any sort of length scale.

I've also added an example of how to calculate native budgets in ECCO V4r4 using xbudget. The notebook calls an example ECCO dataset that I've uploaded to Zenodo. I could move budget_diagnostics.py to that repository and do preprocessing there if you think that would be a better home for it.

[hdrake]

This is great, @anthony-meza!

In general, I would like to separate model-specific preprocessing of budget terms from the core xbudget functionality.

Can you move llc90/budget_diagnostics.py to examples/eccov4r4_budget_diagnostics.py?

With the understanding that we would indeed eventually like to replace the lateral_divergence operation with something more generic that works for any xgcm.Grid with face_connections, this looks good to me as a placeholder!

[anthony-meza]

I agree with your suggestions, this looks a lot better. It should be ready to merge now @hdrake!