Set version to 0.15.0 and update changelog release to today

[argerlt]

Description of the change

Conclusion of the discussion started in #660, accepting the candidate release from #665 as the final minor release 0.15.0.

Progress of the PR

• [N/A] Docstrings for all functions
• [N/A] Unit tests with pytest for all lines
• [N/A] 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.
• Contributor(s) are listed correctly in __credits__ in orix/__init__.py and in
  .zenodo.json.

[hakonanes]

Sorry for being MIA here. Will have more time in two weeks. This looks good, I'd just change the date to today, merge, and release!