Fix bugs in getting plot markers from geometrical EBSD simulation

[hakonanes]

Description of the change

• Line markers from non-allowed (according to kinematical diffraction) kikuchi bands in a geometrical EBSD simulation are returned as white, instead of not being given a colour and thus an IndexError is later thrown (see Provide extra colors in addition to the standard eleven from TSL in GeometricalEBSDSimulation.bands_as_markers() #318)
• Zone axes labels are properly returned when the amount of zone axes is large, instead of throwing an IndexError in another place

Added tests to catch these two cases.

Close #318.

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 doc/changelog.rst.