Implement axis directionality (in low-level ufuncs)

[hdrake]

This is an attempt to revive prior efforts to add a sense of directionality to xgcm grid axes, especially #338.

Unfortunately, @gmacgilchrist's prior attempt at this PR predated the ufunc-ification of grid operations (see PR #479 and the issue it closes #344), which meant that it eventually became stale and was closed.

• Closes [FEATURE] Specify directionality of axes #337
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[hdrake]

@TomNicholas, I started reading through your big ufunc PR and the source code. I would love to take you up on your offer of walking me through the current state of the code, and then maybe you can talk me through how you would approach this PR as a concrete example. Pinging @NoraLoose, @raphaeldussin, and my new postdoc @liu-ran to keep them in the loop.

Easiest to coordinate a call over email?

[hdrake]

I think we can do this in a way that allows for Grid.cumsum and Grid.cumint operations to also be reversed (see #605), picking back up where @jbusecke left off on the stale PR #256.

[TomNicholas]

Sounds good. I'm going to have to remind myself of how all this works 😅

My email is Tom at Earthmover dot io