From Pjotr Svetachov, "have added some missing serializers for RigGeomery. Withouth them I
ran into two issues.

At first you get a bunch of warnings that osg::ComputeBoundCallback
and osg::UpdateCallback were unsupported wrapper classes when
converting fbx models with skeletal animation to osg(t/b).

The second issue was that when reading, the readers fail to read the
ComputeBoundCallback and UpdateCallback and set them to NULL which
messes up the RigGeometry.

Because a RigGeometry makes his own classes in the constructor it
might be preferable to not write them at all, because now those
classes are being made two times when reading a RigGeometry. But after
thinking about this that would place too much limits on them (you
won't be able to share or name them and save that information or make
a new inherited class from them and write that one) So I ended up
thinking the best way was to just write the files.
"