Add EBSD.static_background property storing static background pattern

[hakonanes]

Description of the change

Part of the effort to remove custom metadata from EBSD.metadata (#466).

• Adds a EBSD.static_background property that is None by default, unless a static background could be successfully read from file. This is only the case for Bruker and kikuchipy h5ebsd files and the NORDIF binary file (when background pattern is stored in the same directory as Pattern.dat with the standard name as .bmp file). The property is settable, and a warning (not an error) is raised if the NumPy or Dask array's shape or data type is different from the signal's shape or data type.
• API reference and user guide updated accordingly.
• A warning is added to the user guide (cropping and load/save notebooks) informing the user that the xmap, detector and static_background EBSD properties aren't updated whenever a HyperSpy inherited method that changes the pattern shape and/or data type is called (like inav, isig, rebin etc.). This addresses issues raised in EBSD.xmap property needs vast improvements #227 and Rethinking the EBSD.detector attribute #277 by letting it be up to the user to update the properties...

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 numpy as np
>>> import kikuchipy as kp
>>> s = kp.data.nickel_ebsd_small()
>>> s
<EBSD, title: patterns My awes0m4 ..., dimensions: (3, 3|60, 60)>
>>> s.static_background
array([[84, 87, 90, ..., 27, 29, 30],
       [87, 90, 93, ..., 27, 28, 30],
       [92, 94, 97, ..., 39, 28, 29],
       ...,
       [80, 82, 84, ..., 36, 30, 26],
       [79, 80, 82, ..., 28, 26, 26],
       [76, 78, 80, ..., 26, 26, 25]], dtype=uint8)
>>> s.static_background = s.static_background.astype(np.uint16)
/home/hakon/kode/kikuchipy/kikuchipy/signals/ebsd.py:171: UserWarning: Background pattern has different data type from patterns
  warnings.warn("Background pattern has different data type from patterns")
>>> s2 = s.as_lazy()
>>> s2.static_background
dask.array<array, shape=(60, 60), dtype=uint16, chunksize=(60, 60), chunktype=numpy.ndarray>
>>> s2.compute()
[########################################] | 100% Completed |  0.1s
>>> s2.static_background
array([[84, 87, 90, ..., 27, 29, 30],
       [87, 90, 93, ..., 27, 28, 30],
       [92, 94, 97, ..., 39, 28, 29],
       ...,
       [80, 82, 84, ..., 36, 30, 26],
       [79, 80, 82, ..., 28, 26, 26],
       [76, 78, 80, ..., 26, 26, 25]], dtype=uint16)
>>> s2.remove_static_background()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/hakon/kode/kikuchipy/kikuchipy/signals/ebsd.py", line 548, in remove_static_background
    raise ValueError(
ValueError: Static background dtype_out uint16 is not the same as pattern dtype_out <class 'numpy.uint8'>
>>> s2.static_background = s2.static_background.astype(np.uint8)
>>> s2.remove_static_background()
[########################################] | 100% Completed |  0.2s

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 .all-contributorsrc and the table is regenerated.

[hakonanes]

Will merge after checks pass.