Fix for the particle system.

Fix for the particle system.

The problem was that the change to make FGScenery a standard subsystem caused
the particle OSG group to no longer be inserted into the scene graph.  The
solution was to convert the particle group to be set up as a standard branch of
the scene graph, alongside the terrain, models, aircraft, and interior branches.
The particle system is now set up as part of the init() subsystem function call,
so it is compatible with the new subsystem design.