Split up functions for loading datasets

[willschlitzer]

As mentioned by @seisman in this comment, this splits up the dictionaries for loading functions between the public facing functions and the private ones. It eliminates the need to have the new dataset functions accept **kwargs.

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.

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