Inherit docstrings for bounding_box on subclasses

[embray]

As noted in #4264, when a Model subclass defined the bounding_box property or method, if it has its own docstring that docstring ends up being lost--this is because of how the metaclass swallows the bounding_box member of the subclass and replaces it with the base Model.bounding_box property.
See for example Gaussian2D.

This actually has two negative effects:

1. Gaussian2D.bounding_box.__doc__ displays the docstring for Model.bounding_box, not
   the docstring for Gaussian2D.bounding_box.
2. It is impossible to reference Gaussian2D.bounding_box in Sphinx. It results in a missing reference.
   For example this line was originally a direct reference, but it didn't work. That should be fixed.

This is just a documentation issue, but is worth fixing. I think it can be fixed by slightly going back on some of my changes in #4236 and make it so that each subclass does get its own property object in its __dict__ for bounding_box, rather than just referring directly to Model.property. It's slightly wasteful, but only slightly.