Closest cell fix for `get_closest_gridcell` util function

[elehmer]

Description of PR

This PR changes the behavior of get_closest_gridcell by using a tolerance for how far it will search for a gridcell. The tolerance is derived from the dataset resolution attribute. If the input point is farther than one grid cell away from any of the input data it will return None.

Summary of changes and related issue

388

Relevant motivation and context

See issue for motivation.

Dependencies required for this change?

Fixes # (issue), delete if not necessary

Type of change

• [ x ] 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)
• [ ] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

• [ x ] Tested on HUB
• [ ] Test B

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules

[claalmve]

LGTM! Feel free to implement or ignore the above suggestions, PR is totally good as is too.