Rotation lists from streographic gridding

[pc494]

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name: Rotation lists from streographic gridding
about: As is industry standard we will provide an implementation of rotation list generation that use streographic triangles. See #52

• ready for review and merge?

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Release Notes

major
improvement / bugfix
Summary: Overhaul of streographic functionality

What does this PR do? Please describe and/or link to an open issue.
At a user level this provides a rotation_list that is based on the streographic list, at an internal level this is done by first getting beam directions, and then applying rotations about them, using the 'szxz' trick we tried for get_grid_around_beam_direction, with adjustment illustrated by #53. Theoretically it abuses spherical polar coordinates, as most reduced regions are defined by lines of "longtitude" and the equator, which puts hard limits on theta and phi. The exception is the cubic case.

Work in progress?
If you know there is more to do, make a checklist here:

• Needs better tests
• I need to decide if equal area is worth doing
• I need to fix the cubic case, which currently samples slightly too wide a region
• I need to write the theory out somewhere
• Delete old code that did this
• Make StructureLibrary compliant with all these changes

** Script for checking IPZ figures **
Making use of the headline formulae from https://en.wikipedia.org/wiki/Stereographic_projection#Visualization_of_lines_and_planes

from diffsims.utils.gridding_utils import get_beam_directions
import numpy as np
from matplotlib import pyplot as plt

xyz = get_beam_directions('cubic',5,equal='area')
x = xyz[:,0]
y = xyz[:,1]
z = xyz[:,2]

xs = np.divide(x,1-z)
ys = np.divide(y,1-z)

plt.scatter(xs,ys)

[pc494]

I'm planning completing the other unticked boxes in a separate PR, which means this is ready for review.

[dnjohnstone]

I'm planning completing the other unticked boxes in a separate PR, which means this is ready for review.

Could you have a look at the broken tests to start, please? Meanwhile I'll have a look at the rest of it

[dnjohnstone]

Thanks @pc494 this looks pretty good - but like you got bored around docstring time.

I think a few more comments in the code might be helpful too, but not essential.

This does need a pepstorm - but because that might affect other parts of the code and make review tricky let's leave that for a separate pepstorm only PR.

[dnjohnstone]

I think a brief description here and a "See Also" would be a better solution here.