[argerlt]

Description of the change

1. Adds Vector3d.tensor and Vector3d.tensor_outer
2. Updates test_vector3d.py to better match the class-level organization of other test files and increases overage.

NOTE: this looks like a large update, but 300+ lines are just reformatting existing tests.

I would like to add this to the 0.16 release if possible.

Progress of the PR

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

Minimal example of the bug fix or new feature

# Using the definition of the tensor product as verification:
a = Vector3d.random()
b = Vector3d.random()
c = Vector3d.random()
abc = np.dot((a.tensor(b)[0]), c.data[0])
bca = (b.dot(c) * a).data
abc_outer = np.dot((a.tensor_outer(b)[0]), c.data[0])

# printing an example
print(a,'\n\n',b,'\n\n',a.tensor(b)[0])

>>> Vector3d (1,)
          [[ 0.8429 -0.3101  0.4398]] 
>>> Vector3d (1,)
           [[0.4767 0.1299 0.8694]] 
>>>  [[ 0.40181049  0.10951157  0.73277669]
          [-0.14782216 -0.04028824 -0.26958139]
          [ 0.20967513  0.05714598  0.38238187]]

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]

Going over this now. Seems like a great addition to vector arithmetics!

@argerlt, is it OK if I add an example with vector arithmetics to the gallery, including a section on vector tensor products? This requires a new vector examples section, with vector arithmetics as the first example (and, for now, only).

[hakonanes]

I got the go-ahead off GitHub, so I've added the example. I've also touched up the docstrings and fixed the return type hint.

Just have to go over the tests, then we can merge.

[hakonanes]

@argerlt, feel free to merge this! I may do so within a couple days if you don't.

[argerlt]

I like these additions, and I also really like the new example. Merging now.