Label diffraction spots in plot

[din14970]

---

name: Option to add labels in simulation diffraction spots

Release Notes

extra parameters in diffraction pattern's plot method for drawing miller index labels next to the diffraction spots

What does this PR do? Please describe and/or link to an open issue.
After discussion in #170 I thought I would open this small PR for adding the option to draw the miller indices of spot patterns. With this change it is now possible to do the following:

fig, ax = plt.subplots()
ax.set_aspect("equal")
ax.set_xlim(-2, 2)
ax.set_ylim(2, -2)
simtest.plot(show_labels=True, label_offset=(0, -0.2), 
             label_formatting={"fontsize": 8},
             size_factor=10, ax = ax)

Does this approach what you had in mind @CSSFrancis ?

Apparently there are also two small additions: the option to mirror pattern masks and the update to the stereo triangle for the hex system. It seems this was not merged before, see #168

Are there any known issues? Do you need help?
I would have liked to create a vectorized implementation but have no idea if this is possible with strings. Probably one could do some smart tricks with string formatting, but I couldn't figure out how to do that with the \bar{} on negative indices. If anyone has a better implementation idea feel free to modify.

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

• Needs a test to be added for coverage

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 CHANGELOG.md.

[hakonanes]

This looks like an important "pedagogical" feature!

I would have liked to create a vectorized implementation but have no idea if this is possible with strings. Probably one could do some smart tricks with string formatting

I don't think matplotlib.pyplot.text supports passing more than one set of position and text. Is that what you had in mind? If you find a way to do that, I would be interested, because we have labeling of zone axes in EBSD patterns in kikuchipy (user guide, API, and zone_axes_labels_as_markers() which calls get_text_list()).

[din14970]

@pc494 @hakonanes I think with this minor edit it would be fine for review now

[pc494]

LGTM