Add convenience method to extract grid of EBSD patterns from signal

[hakonanes]

Description of the change

This PR adds two functions:

• EBSD.extract_grid((5, 5)) which extracts a new EBSD signal with 25 patterns evenly distributed in the original signal.
• kikuchipy.signals.util.grid_indices((5, 5), (149, 200)) which returns 25 2D indices into a map of shape (149, 200). This is used in the above EBSD method.

Numerous other small fixes are also done.

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit
• EBSD.extract_grid()
• Add example to gallery

Minimal example of the bug fix or new feature

See example added to gallery:

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.
• New contributors are added to release.py, .zenodo.json and
  .all-contributorsrc with the table regenerated.