Add computing of image quality, chunk/pattern modules, some FFT wrappers ++

[hakonanes]

Description

Adds:

• get_image_quality() (following Krieger Lassen, see docstring for reference), get_dynamic_background(), fft_filter(), and normalize_intensity() to the EBSD class.
• some FFT computation, and filtering routines in kikuchipy.util.barnes, the latter taken from Connelly Barnes' fftfilter.py
• chunk and pattern processing functions as kikuchipy.util.chunk and kikuchipy.util.pattern modules. These were previously private, but are now part of the public API.
• improved kikuchipy.util.Window class, with three windows: modified Hann, lowpass FFT filter, and highpass FFT filter, all used by Wilkinson et al. (2006) "High resolution mapping..." (see reference in docstrings). These work nicely with fft_filter(). Note that this class was previously called Kernel. Also new is that it is subclassing numpy.ndarray.
• remove_dynamic_background() can now create the dynamic background to remove either via a Gaussian filter in real or frequency space, via scipy.ndimage.gaussian_filter() as before, or via the kikuchipy.util.barnes.fft_filter(). The latter is faster.

Renames methods:

• EBSD.static_background_correction -> EBSD.remove_static_background
• EBSD.dynamic_background_correction -> EBSD.remove_dynamic_background
• EBSD.rescale_intensities -> EBSD.rescale_intensity

A Numba dependency is introduced to speed up some computations, like pattern intensity rescaling used in kikuchipy.util.pattern.rescale_intensity, kikuchipy.util.chunk.rescale_intensity, or kikuchipy.signals.ebsd.EBSD.rescale_intensity. In addition, some speed is gained in other routines due to improved use of Dask and a reduction in necessary computations.

Some CSS modifications to the RTD theme has also been done.

The new and changed functionality has been updated in the documentation.

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Progress

• Docstrings updated
• Code is commented, particularly in hard-to-understand areas
• Documentation and/or user guide updated
• All EBSD methods have been tested
• All kikuchipy.util.chunk functions have been tested
• Ready for review!

How has this been tested?

• Many tests have been reworked and updated

[coveralls]

Coverage remained the same at 100.0% when pulling f822bf4 on hakonanes:image-quality into 3597353 on kikuchipy:master.

[hakonanes]

Yes, this is a large PR... But the majority of lines reported are the SVG files for the new logo.