Fix doc build by disallowing numpydoc v1.9.0

[hakonanes]

Description of the change

This PR fixes the documentation build by disallowing numpydoc v1.9.0, thus fixing #570.

There are some other minor changes:

• fix some incorrectly formatted docstrings
• simplify addition of custom pytest markers (now that we have the "slow" marker)
• also renamed the flag to run tests marked with "slow" from "runslow" to "slow", i.e. pytest --slow is the way to do it now
• update type hints in some quaternion module files

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit

Minimal example of the bug fix or new feature

Not relevant as no API changes were made.

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the unreleased
  section in CHANGELOG.rst.
• Contributor(s) are listed correctly in __credits__ in orix/__init__.py and in
  .zenodo.json.

[hakonanes]

Errors are related to me being a bit too fast replacing paths in test fixtures... Will fix tomorrow.