[Opensg-core] Make Node::_sfVolume non-internal field?

[Carsten N. <car...@gm...>]

	Hello all,

I'd like to make Node::_sfVolume a non-internal field and modify the OSB 
loader/writer to write it out in certain cases (see attached patch).

If a volume is marked as static or infinite, written to OSB and read 
back in that information is lost. I see this when loading from OGRE 
.mesh files (which can contain an explicit volume in case the mesh is 
animated to contain all motions the mesh goes through) storing to OSB 
and loading that back in.

Luckily the OSB format is robust enough that we don't have to bump the 
version number - old versions of OpenSG load OSBs written by ones with 
the patch applied and the other way around.

Any objections/comments?

	Cheers,
		Carsten