Use diffsims for electron scattering and diffraction

[hakonanes]

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

Description of the change

• Depend on diffsims (https://github.com/pyxem/diffsims) for scattering/diffraction stuff, namely the ReciprocalLatticePoint class to handle crystal planes, used by the EBSDSimulationGenerator and GeometricalEBSDSimulation classes in kikuchipy.
• Remove the identical ReciprocalLatticePoint class in kikuchipy, and related functionality which is in diffsims. This increases code coverage to 97%!
• Add changelog entry.

Closes #212.

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean style in as per black

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.

[hakonanes]

Will merge after tests pass.