Update cell's __array__ signature for numpy2

Update Cell.__array__() to new numpy specs and add small test of array view/copy behaviour. I didn't actually run this with numpy 2.0.

This MR includes latest master.

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.