WIP: Wrap xyz2grd

[willschlitzer]

This pull request wraps the gmt module xyz2grd.

Fixes #

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 adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[willschlitzer]

I wasn't sure about the etiquette/procedures for taking on another author's pull request (in this case #636), so I opened this PR to wrap xyz2grd.

[weiji14]

@willschlitzer, I would suggest continuing work on that #636 PR instead of starting a new one here. People (especially newcomers) put a lot of work into making a Pull Request and it is best to give them credit for their original work. There could also be reviews in the orginal PR that we want to keep (though not much in this case).

As a maintainer, you should be able to commit directly to that branch, and I would start with a branch update from master (though maybe ping the original author first to check). C.f. #525 where I continued work on someone's old PR after asking for permission.