Use of GPUs

[hakonanes]

We should try to take advantage of GPUs by writing some GPU kernels. I don't have an NVIDIA GPU available, so my choice would be to use PyOpenCL instead of CuPy.

@drowenhorst-nrl have written some kernels in PyEBSDIndex that we could take inspiration from, e.g. the static background subtraction, used in one of the Radon transform functions. Such a kernel could be an alternative to our background subtraction.

In general, I think more per pattern operations in the kikuchipy.pattern module could be replaced by PyOpenCL kernels, like image rescaling. We have CPU acceleration from Numba here, but it would be good to test GPU acceleration as well.

Other resources:

• Python package gputools