Prepare for 0.6rc1 release

[hakonanes]

Description of the change

Release prep:

• Bump version to release candidate 0.6rc1
• Add @CSSFrancis to package credits
• Update README to use format in orix
• Update license year to 2024

Other changes:

• Actually remove the class method StructureLibrary.from_crystal_systems(). The method previously threw a NotImplementedError, but will now throw an AttributeError. Mentioned in the changelog under Removed.
• Replaced deprecated call to Rotation.from_neo_euler() with Rotation.from_axes_angles().

I've cleaned up the test suite a bit:

• Replaced most uses of xfail(error) with with pytest.raises(error):. To me, it seems like best practice is to use the latter when we intentionally raise errors in the code, while the former is mostly used as a temporary measure for bugs that will be fixed.
• Silenced some warnings by making pytest ignoring them
• Close most of "opened" Matplotlib plots in tests using plt.close() (silencing a warning of > 20 open plots from Matplotlib)
• Yield a temporary pickle file to tests via fixture, removing the need for manually removing the file in a test

Progress of the PR

• [x] Docstrings for all functions
• [x] Unit tests with pytest for all lines
• [x] Clean code style by running black
• [x] Wait for #209

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 diffsims/release_info.py and in .zenodo.json.

[hakonanes]

@CSSFrancis, I listed you as a maintainer alongside Duncan, Phillip, and myself in the release info. Which I thought you already were. I hope this is OK?

[hakonanes]

Great and great!

Thanks for looking it over. We should be good to merge and release once your doc examples PR is in.