Remapping Code Coverage, Modularize Reused Code, and Documentation Updates

[aaronzedwick]

Closes #963 #924

Overview

Adds tests to get a more complete code coverage. Takes reused code from nearest_neighbor and inverse_distance and creates one general function for them to call, this should cut down on code and make it easier to create new remapping functions. Updates documentation so the remapping functions are consistent with each other. Also fixes a bug I found when trying to switch from cartesian or spherical after performing one remap.

PR Checklist

General

• An issue is linked created and linked
• Add appropriate labels
• Filled out Overview and Expected Usage (if applicable) sections

Testing

• Adequate tests are created if there is new functionality
• Tests cover all possible logical paths in your function
• Tests are not too basic (such as simply calling a function and nothing else)

Documentation

• Docstrings have been added to all new functions
• Docstrings have updated with any function changes
• Internal functions have a preceding underscore (_) and have been added to docs/internal_api/index.rst
• User functions have been added to docs/user_api/index.rst

[ahijevyc]

Switch from ux.open_dataset to ux.open_grid where possible, replacing destination_dataset.uxgrid with destination_grid afterwards.

Can we avoid hard-coding reconstruct=True when getting the Ball Tree?
This probably needs to wait for Grid.get_ball_tree to be enhanced.

[ahijevyc]

Great switch to open_grid. Thanks for looking into necessity of reconstruct=True