Pooch integration, mark online tests, better handle online-dependent documentation, distributed testing

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR addresses issues raised in Transient data cleanup #183
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.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?

• Marks several tests reliant on external services (online).
• Better typing in call signatures.
• Removes several unused imports.
• Added pytest-xdist to the development dependencies.
• Adding in a pooch registry for verifying file integrity when fetching testing data.

Does this PR introduce a breaking change?

Yes. pytest-xdist is now a development dependency. By default, it is not being used, but it can be enabled by providing positional arguments to the pytest call.

Other information:

I've copied and slightly improved upon the new testing data system currently in #175. I'll handle the merge of the changes to there, but this current system is nearing finalization. The last thing that I'm waiting on are some changes from @RondeauG to xhydro-testdata so that I can make a dated tag that will become the default testing data branch for xhydro (updated whenever there are additions to xhydro-testdata). This will help prevent breakages from a moving main branch over there.

[Zeitsperre]

I need to convert the exiting pooch calls to the new system, but before that, the pooch-based setup code causing errors at the moment needs to be converted to pytest fixtures. Will tackle that tomorrow.

[sebastienlanglois]

@Zeitsperre I have implemented the fix for the GIS module here : #213

[sebastienlanglois]

Impressive work, thanks for improving the quality of the codebase!

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[RondeauG]

We'll see if the GIS notebook passes. Otherwise, we might need to run the outputs locally as was done before.

(Note that I didn't include that notebook in #211, since it wasn't run by RtD).