Deprecate generators.VirtualBSEGenerator in favor of imaging.VirtualBSEImager

[hakonanes]

Description of the change

kikuchipy.generators.VirtualBSEGenerator is deprecated in favor of kikuchipy.imaging.VirtualBSEImager. The deprecated class and the generators module will be removed in version 0.9.

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

>>> import kikuchipy as kp
>>> s = kp.data.nickel_ebsd_small()

# Before

>>> vbse_gen = kp.generators.VirtualBSEGenerator(s)

# Now
>>> vbse_imager = kp.imaging.VirtualBSEImager(s)

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.