Add 'const' qualifier to getRootNode() in OctreeShape class
Revise signature of setOctree() in (Osg)OctreeRepresentation

Use a const reference to OctreeShape instead of a const reference of a
shared pointer to OctreeShape.
Build a OSG node for OctreeNode no matter if it is active or not.

A OctreeNode may be activated later even if it is inactive in the
beginning.

Also, revise the implementation of setOctree() since its signature has
changed.
Update unit test and render test for OsgOctreeRepresentation.
Add a const version of  getRootNode() in OctreeShape class