Use xarray for OPeNDAP requests

[nate-kean]

Description

This is a small change that will make the codebase more consistent. RAiDER uses several packages for interfacing with datasets in different places, and one is pydap for GMAO and MERRA2 data. This change changes pydap out for xarray, which is already used in many other places.

For GMAO, pydap is no longer used at all. For MERRA2, since it requires authentication, pydap is still used under the hood through xarray's pydap engine.

Motivation and Context

How Has This Been Tested?

All tests still pass. I realized the path for fetching GMAO data before T0 was not tested in test_downloaders, so I added a test for it.

Screenshots (if appropriate):

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have added an explanation of what your changes do and why you'd like us to include them.
• I have written new tests for your core changes, as applicable.
• I have successfully ran tests with your changes locally.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.