gh-1052: RadialWindow is now a dataclass

[paddyroddy]

Description

Just spotted this while preparing for the ARCHER2 talk.

Closes: #1052

Checks

• Is your code passing linting?
• Is your code passing tests?
• Have you added additional tests (if required)?
• Have you modified/extended the documentation (if required)?
• Have you added a one-liner changelog entry above (if required)?

[ntessore]

Well spotted!