Rename boundary to padding.

[jbusecke]

Got really inspired by #557 and decided to try to build on top of that with a PR that renames boundary to padding (which makes it consistent with the nomenclature we discussed, and is already used in the padding.py module). This should lead to less confusion over the fact that we do not in fact implement physical boundary conditions yet.

This worked pretty well, except for a bunch of errors which all seem to have the same source.

FAILED xgcm/test/test_grid.py::test_dask_vs_eager[yperiodic_2d_left-extend-diff] - TypeError: diff_center_to_right() got an unexpected keyword argument 'padding'

There must be some place in the grid_ufunc internals where I have missed to rewire the logic to take padding instead of boundary. @TomNicholas, do you have a hunch what could be going on?

• Fix remaining tests failures
• Rebase/Merge onto main once Remove computation methods on Axis class #557 is merged.
• Make sure to rename all occurences/sections dealing with boundary/padding (maybe also finish Vector padding docs (fresh try) #476)

---

• Towards Plans for upcoming breaking changes (maybe v1.0.0?) #414
• 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