Allow user to interact with BoundingBoxTree coordinates

[ordinary-slim]

Describe new/missing feature

I am working with mesh motion and would like to avoid recomputation of BoundingBoxTrees. In my case, the mesh motion is a simple translation so I would like to be able to shift the _bbox_coordinates of BoundingBoxTree by a 3D vector. The issue is that it is a private member of the class with no Python exposure.
Making the attribute protected instead of private would allow a user to inherit from the class and interact with it from there. Another option would be to expose it to Python the same way the mesh coordinates are exposed.

Suggested user interface

No response

[jhale]

The latter option (exposing the coordinates) seems like the consistent approach. The docstring needs to reflect that this does not update the underlying tree.