Simplify the Rotation.from_euler() API

[pc494]

Description of the change

See discussion in #215

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.

close #215

[hakonanes]

Great that we can remove code but not functionality! I just have a couple questions I would like answered if you find the time.