Add virtual backscatter electron image generator

[hakonanes]

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

Description

Adds a VirtualBSEGenerator, following the workflow in pyxem (https://github.com/pyxem/pyxem/blob/master/pyxem/generators/vdf_generator.py). The generator is initialized with an EBSD or LazyEBSD signal, and has methods to get a virtual backscatter electron (BSE) RGB image or a stack of images from a detector grid, (5, 5) default. The grid can also be plotted, and three tiles/regions of interest (ROIs) in the grid can be coloured red, green and blue if desired, to indicate which tiles were used to generate an RGB image.

It generates a VirtualBSEImage signal object, which is introduced in this PR. This is a thin wrapper around HyperSpy's Signal2D. This class has no lazy implementation, which is considered unnecessary for now.

Also renames EBSD.virtual_backscatter_electron_imaging to EBSD.virtual_bse_imaging and EBSD.get_virtual_image to EBSD.get_virtual_bse_image.

Checklist

• Docstrings for all functions
• Update virtual imaging doc page
• Add generator doc page
• 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.random.random(1600).reshape((2, 2, 20, 20)))
# Initialize generator
>>> vbse_gen = kp.generators.VirtualBSEGenerator(s)
# Set grid shape and plot superimposed on a pattern
>>> vbse_gen.grid_shape = (4, 4)
>>> p = vbse_gen.plot_grid()
>>> p
<EBSD, title: , dimensions: (|20, 20)>
# Get ROI from grid row and column (from upper left)
>>> roi22 = vbse_gen.roi_from_grid(2, 2)
>>> roi22
RectangularROI(left=5, top=0, right=10, bottom=5)
>>> roi00 = vbse_gen.roi_from_grid(0, 0)
>>> roi10 = vbse_gen.roi_from_grid(1, 0)
>>> vbse_imgs = vbse_gen.get_images_from_grid()
>>> vbse_imgs
<VirtualBSEImage, title: , dimensions: (4, 4|2, 2)>
>>> vbse_rgb_img = vbse_gen.get_rgb_image(
    red=roi00,  # Can also pass grid tile indices (0, 0)
    green=roi10,  # (1, 0)
    blue=roi22,  # (2, 2)
)
>>> vbse_rgb_img
<VirtualBSEImage, title: , dimensions: (4, 4|2, 2)>
>>> vbse_rgb_img.data.dtype
dtype([('R', 'u1'), ('G', 'u1'), ('B', 'u1')])

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 2251be0 on hakonanes:improve-virtual-imaging into 6df76a5 on kikuchipy:master.

[coveralls]

Coverage decreased (-2.5%) to 97.523% when pulling a301f24 on hakonanes:improve-virtual-imaging into 6df76a5 on kikuchipy:master.