Add plane model.

include/smCollision/smPlaneCollisionModel.h

include/smCore/smGeometry.h

include/smGeometry/smPlaneModel.h

src/smCollision/smPlaneCollisionModel.cpp

src/smGeometry/CMakeLists.txt

src/smGeometry/smPlaneModel.cpp

src/smSimulators/smVegaFemSceneObject.cpp