Numpy-like flattening

[argerlt]

Note: force-pushed on April 9th to remove unnecessary reformatting.

Working but unfinished answer to #595. Implementation is complete, docstrings still need fixing though, and this causes pytest to raise 191258 warnings. This is to be expected, but I want to slim that number down before merging.

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

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.

[argerlt]

pre-commit.ci autofix

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