Bounding Volumes can be automatically calculated

[Pherakki]

Provide details on the proposal. Model bounding volumes can be automatically calculated after all:

• Bounding box dims: Bounding box of mesh bounding boxes and scenegraph node positions
• Bounding sphere centre: Average of mesh bounding box centres
• Bounding sphere radius: Distance between the sphere centre and whichever of the two model bounding box corners are furthest away

To accommodate this, the "Export Bounding Box/Sphere" checkboxes can be changed to drop-downs that have None, Manual, and Automatic options. Manual should allow the current manual editing of bounding boxes, with the option to fill in these from automatically calculated values, and Automatic should create the bounding volumes in the same fashion as the original volumes on export.

[Pherakki]

Implemented in d7da89f2a31c9a6d7af5dfb87eb7397840076491