Create vectors from path ends

[hakonanes]

Description of the change

• Add class method to obtain vectors drawing a path between two vectors on the sphere. This is useful when delineating a path or the border of a spherical region in the stereographic projection or in 3D.
• Make the polar stereographic grid use Matplotlib's grid.alpha parameter
• Add an example showing how to combine rotations. This example is the same as in the paper by Rowenhorst et al. (2015) showing the difference between active and passive rotations. In that paper they use mainly passive rotations. We do the same in orix. This means rotations are combined from left to right, as in rAB = rA * rB, where rotations are done in A, B order.

These changes were originally added in #442. The main change in that PR has to wait for other changes before we can continue with it, so I'm making separate PRs for the changes that we can merge now.

Progress of the PR

• [x] Docstrings for all functions
• [x] Unit tests with pytest for all lines
• [x] Clean code style by running black via pre-commit

Minimal example of the bug fix or new feature

See the new example on combining rotations to see all the changes in this PR in action: https://orix--445.org.readthedocs.build/en/445/examples/rotations/combine_rotations.html

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.

[hakonanes]

Thank you for reviewing!