Improve check for bbox

[oscargus]

PR summary

Passing something else than a Transform would lead to an AttributeError, but now the better error message should show up.

PR checklist

• [N/A] "closes #0000" is in the body of the PR description to link the related issue
• [N/A] new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines

[jklymak]

Doc build failed?

[QuLogic]

Yes, everything failed earlier this morning due to network issues. I've restarted them all.

[anntzer]

Seems more natural to just check_isinstance(BboxBase, bbox=bbox) if we want to go that direction, no? (Also consistent with the line immediately after.)
Feel free to dismiss if you disagree.

[oscargus]

Seems more natural to just check_isinstance(BboxBase, bbox=bbox) if we want to go that direction, no? (Also consistent with the line immediately after.) Feel free to dismiss if you disagree.

Except that a TypeError will be raised. Which on the other hand makes more sense. I can change if there is agreement on this.

Edit: this also raises the question if is_bbox is actually required? If this is changed, it will be unused. So I'll deprecate it as well if I change it.

[oscargus]

I added the change as a second commit. Not sure if I should deprecate it as a property or classproperty though.

Is addressed.