Vector tensor product

[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.