Testing data fetching via pooch

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGES.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Added pooch as a hard dependency (this can be revisited)
• Added xhydro.testing.helpers.py and xhydro.testing.registry.txt
• Added functions:
  • generate_registry: Parses data found in package (xhydro.testing.data), and adds it to the registry.txt
  • load_registry: Loads installed (or custom) registry and returns dictionary
  • populate_testing_data: Fetches the registry and optionally caches files at a different location (helpful for pytest-xdist)
• Added a pre-commit hook for validating NumPy docstrings, fixed a few docstrings.

Does this PR introduce a breaking change?

Yes. Testing data management is largely handled at the environment variable level:

• XHYDRO_DATA_DIR can be used to override the default cache location set for pooch (on *nix: $XDG_CACHE_HOME; Must be an absolute path)
• XHYDRO_TESTDATA_BRANCH can be used to override the branch that the testing data
  should be fetched from (default: main).

Other information:

https://www.fatiando.org/pooch/latest/sample-data.html
Based on a similar approach taken for: https://github.com/Ouranosinc/xclim-testdata

Typically, the loaded pooch registries are given dog names, so I landed on DEVEREAUX as an appropriate name for this project.

[RondeauG]

Hard to really test or review without data in xhydro-testdata, but overall this seems good!

[RondeauG]

LGTM!