update default units handler to use explicit cache folder

Closes #253

Changes the default unit registry cache to hydrotools/pint_cache. Added a new unit handler test to make sure pint actually uses the cache.

Checklist

[x] PR has an informative and human-readable title
[x] PR is well outlined and documented. See #12 for an example
[x] Changes are limited to a single goal (no scope creep)
[x] Code can be automatically merged (no conflicts)
[x] Code follows project standards (see CONTRIBUTING.md)
[x] Passes all existing automated tests
[x] Any change in functionality is tested
[ ] New functions are documented (with a description, list of inputs, and expected output) using numpy docstring formatting
[ ] Placeholder code is flagged / future todos are captured in comments
[ ] Reviewers requested with the Reviewers tool :arrow_right: