Improve testing of two Numba functions

[hakonanes]

Description of the change

• Improve tests of private Numba functions. With these additions, Orientation refinement produces rubbish results on some Windows installations #494 and the 0.5.7 patch release could have been avoided.
• Stress in the contributing guide that Numba functions must be tested both by calling their py_func() for coverage and their machine code by calling them directly! This would have caught the case where a Numba function gave different results on Windows and Ubuntu from the same Python (NumPy) code.
• Move contributing guide and code of conduct to the top directory to increase visibility.

Progress of the PR

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

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.
• New contributors are added to .all-contributorsrc and the table is regenerated.