Fix scaling of region of interest coordinates in virtual backscatter electron imaging generator

[hakonanes]

Description of the change

• Fix scaling of region of interest coordiantes in the virtual backscatter electron imaging generator, i.e. multiply with the detector pixel size as set in EBSD.axes_manager["dx"].scale and EBSD.axes_manager["dy"].scale
• Close EBSD signal axes not set correctly in the Bruker h5ebsd reader #346 and TraitError from s.get_virtual_bse_intensity(roi) for EBSD signal read from Bruker h5ebsd #347
• Use nansum in virtual backscatter electron imaging

Thanks a lot to @Fred-Ad for opening those issues!

Progress of the PR

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

Minimal example of the bug fix or new feature

>>> import kikuchipy as kp
>>> import hyperspy.api as hs
>>> s = kp.data.nickel_ebsd_large()
>>> print(s.axes_manager["dx"].scale, s.axes_manager["dy"].scale)
1.0 1.0
>>> s.axes_manager["dx"].scale = 560  # 70 um pixel size, binning of 8
>>> s.axes_manager["dy"].scale = 560
>>> vbse_gen = kp.generators.VirtualBSEGenerator(s)
>>> vbse_gen.plot()  # Grid tile coordinates are displayed correctly
>>> roi = hs.roi.RectangularROI(left=0, top=0, right=10 * 560, bottom=10 * 560)
>>> s.plot_virtual_bse_intensity(roi)  # Doesn't throw TraitError
>>> vbse_rgb = vbse_gen.get_rgb_image(r=(2, 1), g=(2, 2), b=(2, 3))  # Doesn't throw ValueError
>>> vbse_rgb.plot()

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.