Provide default OBB tag name in header

[Patrick Shriwise]

The default vale of the tag name used to store box data on EntitySets representing nodes of the OBBTree is "OBB". This is currently defined in OrientedBoxTreeTool.cpp rather than in the header and cannot easily be accessed by applications outside of MOAB.

For instance, I'm working with a program which modifies a given mesh and then writes the new mesh to a new filename. If an OBB already exists, then it will become out of date. I'd like to be able to check for the existence of an OBB tree and remove the tree using a MOAB-defined default tag name.