Shift of management of the precipitation OSG scene graph to the scenery manager.

Shift of management of the precipitation OSG scene graph to the scenery manager.

The precipitation branch of the OSG scene graph is now centrally managed by the
scenery manager rather the precipitation manager itself.  The scene graph
initialisation from the constructor has also been shifted into the new
FGPrecipitationMgr::sceneGraphSetup() function.