Improve plotting of vector labels

[hakonanes]

Description of the change

This PR improves plotting of vector labels in the stereographic projection by:

• Adding convenience function orix.plot.format_labels(v, brackets=("", ""), use_latex=True) accepting an array of vectors and returning an array of strings, one integer label per vector.
• Adding an offset=None parameter in StereographicPlot.text() to control offset in stereographic coordinates (X, Y). When plotting with Vector3d.scatter(..., vector_labels=labels), this offset can be passed via text_kwargs={"offset": (0, 0.02)}".

This makes labeling a pole figure much easier and nicer.

We can consider "extending" the vector_labels in Miller.scatter() by allowing it to be True, which then automatically creates the labels and adds them accordingly with the correct brackets (hkl/uvw). The danger is that this labeling assumes integer coordinates which might not be true for all vectors. If we add this, a warning should be raised in the case where rounding introduces an error in the coordinate labels above a certain threshold.

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit
• Account for Miller-Bravais (hkil) and Weber [UVTW]

Minimal example of the bug fix or new feature

Create three sets of direct lattice vectors <uvw> and plot them in the stereographic projection with labels and one color per family of directions

from orix import plot
from orix.crystal_map import Phase
from orix.vector import Miller


h = Miller(uvw=[[0, 0, 1], [1, 1, 1], [1, 1, 0]], phase=Phase(point_group="m-3m"))
h2, idx = h.symmetrise(unique=True, return_index=True)

h2.scatter(
    c=np.array([f"C{i}" for i in range(h.size)])[idx],
    vector_labels=plot.format_labels(h2.uvw, ("[", "]")),
    text_kwargs=dict(offset=(0, 0.02), bbox=dict(fc="w", ec="none", pad=0, alpha=0.75))
)

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the unreleased
  section in CHANGELOG.rst.
• Contributor(s) are listed correctly in __credits__ in orix/__init__.py and in
  .zenodo.json.

[pc494]

Sorry @hakonanes, I should have reviewed this. Can I get it sometime next week once the conflicts (also my fault) have been resolved?

[hakonanes]

No problem, sounds good! Will fix the conflicts.

[hakonanes]

By 'no problem', I mean thanks for reviewing :)

[hakonanes]

Conflicts fixed, should be ready for review!

[harripj]

Sorry for the delay @hakonanes, this change looks great and will be helpful to users wanting to improve their figures. Would you mind if I make a very minor change to one of the notebooks just to add offset to one of the plots?

[pc494]

This looks good to me, I'll check @harripj got his change in and then mege.

[harripj]

@pc494 feel free to merge!

[hakonanes]

Great, thank you guys for having a look! Will merge once the Python 3.7 build succeeds.