Set up pattern centre, detector and gnomonic projection classes

[hakonanes]

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

Description

This is an attempt to establish traditional EBSD reference systems for the signal and navigation axes, based upon work made available in the paper Britton, T. B., Jiang, J., Guo, Y., Vilalta-Clemente, A., Wallis, D., Hansen, L. N., Winkelmann, A., Wilkinson, A. J.: Tutorial: Crystal orientations and EBSD - Or which way is up?, Materials Characterization 117,113–126, 2016, doi: https://doi.org/10.1016/j.matchar.2016.04.008.

I imagine the final product of this PR being plotting plane traces as line markers superimposed on EBSD patterns. We'll see how it goes.

It is a work in progress.

Checklist

Will add more actions as the work comes along.

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean style in as per black
•  Rename to gnomonic projection...

Minimal example of the bug fix or new feature

Will add as the work comes along.

>>> import kikuchipy as kp
>>> import numpy as np
>>> s = kp.signals.EBSD(np.zeros((10, 10, 10, 10)))
>>> # Your new feature...

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 decreased (-1.5%) to 98.464% when pulling f4771e4 on hakonanes:gnomic-projection into d82a939 on kikuchipy:master.

[hakonanes]

Let's call it gnomonic projection as well...

[hakonanes]

All of this will be implemented in #204.