GMTDataArrayAccessor: Support passing values using enums GridRegistration and GridType for grid registration and type

[seisman]

Description of proposed changes

This PR improves the GMTDataArrayAccessor by allowing setting grid registration and gtype using more readable enums, rather than numeric values.

Refer to #499 (comment) for the initial thoughts but implemented using enums as proposed in #499 (comment).

This PR does a few things:

1. 8c7f8f6: Add enums GridReg/GridType.
2. 94086f7: Update GMTDataArrayAccessor to support passing enum values (e.g., grid.gmt.registration=GridReg.PIXEL, not grid.gmt.registration=1).
3. 8dd3d5f: Update the existing tests to check grid registration and gtype using enums (e.g., assert grid.gmt.registration==GridReg.PIXEL, not assert grid.gmt.registration==1
4. fd099b0, f0b30ab: Update the tests in test_accessor.py

The plan is to cherry-pick the commit 8c7f8f6 into a separate PR and keep the other commits in this PR, so that we can have two separate entries in the release changelog. [Done in #3693]

Please note that:

1. Changes in this PR are backward-compatible.
2. String-type values (e.g., grid.gmt.registration = "gridline") initially proposed in Make a gmt xarray accessor to store metadata from grdinfo #499 (comment) are not supported in this PR. It's simple to support string-type values, but I feel it's not necessary (see the reverted changes in 259771a).

TODO:

• Add enums GridReg/GridType to API docs [in Add enums GridRegistration and GridType for grid registration and type #3693]

Preview: https://pygmt-dev--3696.org.readthedocs.build/en/3696/api/generated/pygmt.GMTDataArrayAccessor.html

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.