Add NotImplementedError to RGB VirtualBSEImage rescale/normalize

[hakonanes]

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

Description

Raise NotImplementedError when trying to call rescale_intensity() and normalize_intensity() on a VirtualBSEImage object with a HyperSpy RGB data type.

Also improve some doc wording.

Checklist

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean style in as per black

Minimal example of the bug fix or new feature

>>> import kikuchipy as kp
>>> import numpy as np
>>> s = kp.signals.EBSD(np.zeros((10, 10, 10, 10)))
>>> vbse_gen = kp.generators.VirtualBSEGenerator(s)
>>> rgb = vbse_gen.get_rgb_image(r=(0, 0), g=(0, 1), b=(0, 2))
>>> rgb.rescale_intensity()
[...]
    raise NotImplementedError(
NotImplementedError: Use RGB channel normalization when creating the image instead.

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.

[coveralls]

Coverage remained the same at 100.0% when pulling 2c131f9 on hakonanes:improve-doc-wording into 2fbf2ff on kikuchipy:master.

[coveralls]

Coverage remained the same at 100.0% when pulling b52a7c9 on hakonanes:improve-doc-wording into 2fbf2ff on kikuchipy:master.