Make adaptive histogram equalization available to all classes

[hakonanes]

Description of the change

Made adaptive_histogram_equalization() available to both master pattern classes and the VirtualBSEImage class. Also added an example to the docs showing the use of these methods.

Progress of the PR

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

Minimal example of the bug fix or new feature

See example in the docs built from this PR.

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.