Add function to load Blue Marble dataset

[willschlitzer]

This PR (hopefully) adds a function to load the Blue Marble image dataset.

• Official documentation: https://www.generic-mapping-tools.org/remote-datasets/earth-daynight.html
• Recipe: https://github.com/GenericMappingTools/gmtserver-admin/blob/master/recipes/earth_day.recipe
• Preview: https://pygmt-dev--2235.org.readthedocs.build/en/2235/api/generated/pygmt.datasets.load_blue_marble.html

Addresses #1442

Reminders

• [x] 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.
• [x] 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.

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

[weiji14]

Gonna push some commits here to get the load_blue_marble function in for PyGMT v0.12.0.