Restructure API, add two subpackages pattern and filters

[hakonanes]

Description

Restructures the API, adding two subpackages:

• kikuchipy.pattern. The single pattern processing used indirectly by the EBSD class is directly available from the subpackage. It also has two modules chunk and correlate, which expose the chunk processing used directly by the EBSD class, and for now only one method to correlate patterns (normalized cross-correlation), respectively.
• kikuchipy.filters, containing methods to get transfer functions, spatial kernels etc. to do frequency domain filtering, neighbour pattern averaging etc.

The documentation, docstrings and tests have been updated accordingly.

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
>>> kp.pattern.__all__
['chunk',
 'correlate',
 'fft',
 'fft_filter',
 'fft_frequency_vectors',
 'fft_spectrum',
 'get_dynamic_background',
 'get_image_quality',
 'ifft',
 'normalize_intensity',
 'remove_dynamic_background',
 'rescale_intensity']
>>> kp.filters.__all__
['distance_to_origin',
 'highpass_fft_filter',
 'lowpass_fft_filter',
 'modified_hann',
 'Window']
>>> kp.signals.__all__
['EBSD', 'EBSDMasterPattern', 'LazyEBSD', 'LazyEBSDMasterPattern', 'util']

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 f097e77 on hakonanes:restructure-api into 5032787 on kikuchipy:master.