Add function to expand asymmetric unit cell of Phase

[viljarjf]

Description of the change

Adds Phase.expand_asymmetric_unit(), which adds all symmetrically equivalent Atom entries into Phase.structure.
This already exists in diffsims (ReciprocalLatticeVector.sanitize_phase(), with a #TODO to upsteam into orix. However, I don't think the implementation in diffsims is entirely correct, seeing as the discrepancy between diffsims' and diffpy's axis alignment convention does not seem to be accounted for.

This expansion seems to be performed automatically when reading from a cif which includes symmetry information. I added a test for this, which includes the cif-file test-cases as raw strings. If there is another preferred way to do this please let me know. I saw the cif_file-fixture, but I felt making these single-use files into fixtures was unnecessary.

Additionally, I found Structure.placeInLattice(), which does the same as #469 for both coordinates and ADPs.

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

>>> phase = Phase(
      structure=Structure(
          atoms = [Atom("Si", xyz=(0, 0, 1))],
          lattice=Lattice(4.04, 4.04, 4.04, 90, 90, 90)
      ),
      space_group=227,
  )
  >>> phase.structure
  [Si   0.000000 0.000000 1.000000 1.0000]
  >>> phase.expand_asymmetric_unit()
  >>> phase.structure
  [Si   0.000000 0.000000 0.000000 1.0000,
   Si   0.000000 0.500000 0.500000 1.0000,
   Si   0.500000 0.500000 0.000000 1.0000,
   Si   0.500000 0.000000 0.500000 1.0000,
   Si   0.750000 0.250000 0.750000 1.0000,
   Si   0.250000 0.250000 0.250000 1.0000,
   Si   0.250000 0.750000 0.750000 1.0000,
   Si   0.750000 0.750000 0.250000 1.0000]

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.