Document EBSD reference frames

[hakonanes]

Signed-off-by: Håkon Wiik Ånes hwaanes@gmail.com

Description of the change

Closes #208 and #211.

Progress of the PR

• Docstrings for all functions with examples!
• Unit tests with pytest for all lines in ebsd_detector.py
• Clean style in as per black
• Double check conversions between vendor projection center conventions (Jackson et al. (2019)).
• Continue on the reference frame part of the documentation, with clear explanations of how to use the EBSDDetector class
• Add detector stuff to the API

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  the unreleased section in doc/changelog.rst.