Silence diffpy structure 3.4.0 warnings

[hakonanes]

Description of the change

Solves #643.

Almost. One source of warning comes from diffpy.structure itself. Too deep to catch with a simple warning filter on our end. I'll open a PR to diffpy.structure to silence it: diffpy/diffpy.structure#201.

Progress of the PR

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

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.