Add function for accessing MERRA-2

[kandersolar]

• [ ] Closes #xxxx
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• 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`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
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I found a simple way to retrieve MERRA-2 data without needing extra dependencies like xarray and pydap (#1274). It is a little slow, but seems to work well otherwise.

Note, the tests should skip until I add the necessary credentials to the repository's secrets. credentials now added

[AdamRJensen]

Missing a test where the start/end dates have a timezone.

[IoannisSifnaios]

I guess you are maybe missing a "bad request" test? E.g., if username/password do not exist?

Otherwise LGTM! :-)

[adriesse]

Could there be links to the above three data sets here? I couldn't see how to get there from this link.

[adriesse]

Would it be possible to include all these in the variable map? And make that visible in the documentation?

Also, if there is down-welling long-wave, please add it for module temperature modeling.

[adriesse]

I found a simple way to retrieve MERRA-2 data without needing extra dependencies like xarray and pydap (#1274). It is a little slow, but seems to work well otherwise.

Is there anything worth describing about this technique? Is it a work-around of some sort?