Deprecate map_into_symmetry_reduced_zone() in favor of reduce()

[argerlt]

Description of the change

#558 was having a bad case of scope creep, so I'm breaking it up into some concise additions.

Additionally, some recent folder renaming and style changes meant a rebase made fore a confusing commit history.

This PR just depreciated map_into_symmetry_reduced_zone to reduce. Some clarifying code comments
were added, but other than that functionality is identical to before.

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

>>> from orix.quaternion.symmetry import C4, C2
>>> data = np.array([[0.5, 0.5, 0.5, 0.5], [0, 1, 0, 0]])
>>> M = Misorientation(data)
>>> M.symmetry = (C4, C2)```

Before:
>>> M.map_into_symmetry_reduced_zone()

After:
>>> M.reduce()

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• [n/a] 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.

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[hakonanes]

Thanks for making this PR!

I added a few extra internal improvements. I'd appreciate it if you could glance over them before merging.