Use memory efficiently, GNSS geometric heights

[jlmaurer]

Description

xarray makes available two methods for opening datasets, open_dataset and load_dataset. Using open_dataset allows for lazy loading and memory efficiency. This PR change all the locations were load_dataset was used to `open_dataset', resulting in much better memory performance for RAiDER. This PR also wraps netcdf dataset opening with try/except/finally logic that ensures the datasets are closed properly.

In addition, the CDS API call for ERA-5 model levels was doubling calls to the API because the variables were in two pairs. I think this was a holdover from a previous API version. Now all four variables (lnsp, z, t, and q) are called at once.

Finally, GNSS heights were not getting converted to orthometric heights prior to interpolation in RAiDER. This PR implements a conversion from ellipsoid heights to the geoid only for GNSS stations.

All unit tests pass locally.

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.

[jlmaurer]

@sssangha I think this PR is still good to go if you are able to review it. The integration tests that we were talking about are in #805

[jlmaurer]

Closing this and replacing the memory-efficiency part with #808, still working on the geoid correction part.