JOSS Review Improvements

[CDJellen]

Summary of core changes:

• Removed netCDF4 dependency in favor of xarray and scipy.
• Improved tests.

Minor changes and bug fixes:

• Fixed the sign of the longitude in the example notebook.

Future improvements:

• Include station metadata in single-station get_data calls when use_opendap=True.
• Possibly rename the flags and methods given the migration off of netCDF4.