Add retrieval function for NASA MERRA2 reanalysis data

[AdamRJensen]

• ~~[ ] Closes #xxxx~~
• [x] I am familiar with the contributing guidelines
• [x] Tests added
• [x] Updates entries to docs/sphinx/source/api.rst for API changes.
• [x] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
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• [ ] Pull request is nearly complete and ready for detailed review.
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MERRA-2 is the reanalysis dataset provided by NASA and provides hourly aerosol and irradiance data for the entire world starting in 1980. While the irradiance data is of questionable accuracy, the aerosol data is very useful for a number of different clear sky models.

The proposed function retrieves MERRA-2 data from the NASA GES DISC repository.