Wrap grdlandmask

[willschlitzer]

This pull request wraps the grdlandmask module.

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.

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[willschlitzer]

I'm running into a newfound problem with this module. When I run:

import pygmt
new_grid = pygmt.grdlandmask(increment=1, region = [-5, 5, -5, 5])

I receive: TypeError: 'module' object is not callable

I've done the typical trouble shooting (confirmed I'm in the correct conda environment, have the appropriate git branch activated, checked the import statements and function to make sure it's in line with other modules, and, of course, shut it down and started it up again) so I'm not sure why I'm getting this issue. @weiji14 @seisman any ideas?

[seisman]

@willschlitzer Feel free to merge this PR after all tests pass.

[weiji14]

Just some final suggestions. Feel free to merge after the tests pass.