Add enums GridFormat for GMT grid format ID

[seisman]

Description of proposed changes

This PR is a subset of PR #3128.

This PR adds the pygmt.enums module to defined enumerations that can be used in PyGMT. The source layout is inspired by the rasterio package (https://github.com/rasterio/rasterio/blob/main/rasterio/enums.py).

Currently, only enums for GMT grid format IDs are added. They're originally defined at https://github.com/GenericMappingTools/gmt/blob/7809736ba32d87a4a96b15444419eb176c6a35f3/src/gmt_grdio.h#L70. Enums for grid registrations and gtypes are likely to be added when addressing #499 (comment).

This PR also fixes the issue originally pointed by 5888e10.

Preview: https://pygmt-dev--3449.org.readthedocs.build/en/3449/api/generated/pygmt.enums.GridFormat.html

After ea1cc79, the docs look like below:

After ac6239f, the docs look like below:

[seisman]

Ready for review although the documentation doesn't look correct. Maybe we need to follow https://github.com/rasterio/rasterio/blob/9953b28225db3b01193c94b1442d34b828d374aa/docs/api/rasterio.enums.rst. Will explore it later.

[seisman]

Still, I wouldn't say I like how the enums are documented at https://pygmt-dev--3449.org.readthedocs.build/en/3449/api/generated/pygmt.enums.GridFormat.html

[weiji14]

Still, I wouldn't say I like how the enums are documented at https://pygmt-dev--3449.org.readthedocs.build/en/3449/api/generated/pygmt.enums.GridFormat.html

Do we need to have this GridFormat enum in the API docs actually? I don't think users will use it much, it's not like you can pass it into any function/method as an argument.

I'm also ok with keeping it as is right now, and improving the style/layout later. It doesn't look too bad to me.

[seisman]

Do we need to have this GridFormat enum in the API docs actually? I don't think users will use it much, it's not like you can pass it into any function/method as an argument.

Yes for GridFormat, but I think we need to document GridRegistration and GridType (suggested in #499 (comment)).

I'll revert the changes for docs now and explore how to make it better later.

I also prefer to splitting this PR into two smaller PRs, one for adding the GridFormat enums and one for fix the CF-Convention attribute, so that we can have two separate entries in the changelog.

[seisman]

#: is the special way to document attributes, following https://www.sphinx-doc.org/en/master/usage/extensions/autodoc.html#directive-autoproperty.