paramters in its constructor to allow it to be used to create rotations around a point.
This provides the same interface and functionality as osgUtil::TransformCallback but
has the advantage and AnimationPathCallback is fully supported by the .osg and .ive file formats.
leaving it up to the Viewer to specify the mode (which by default is MultiThreaded).
Added a check for the presence of osgParticle systems so that threading is
disabled in this case.
