Use memory efficiently when calling xarray

[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 change results 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.

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.