Fix errors regarding volume of sliced form factors and added Python example

Core/HardParticle/FormFactorTruncatedSphere.cpp

Core/HardParticle/FormFactorTruncatedSpheroid.cpp

Core/Multilayer/Layer.cpp

Core/Particle/Particle.cpp

...lation/ex06_Miscellaneous/HalfSpheresInAverageTopLayer.py