Added topZ method to all IFormFactors

...DecoratedFormFactor/FormFactorDecoratorPositionFactor.cpp

Core/DecoratedFormFactor/FormFactorDecoratorPositionFactor.h

Core/DecoratedFormFactor/FormFactorDecoratorRotation.cpp

Core/DecoratedFormFactor/FormFactorDecoratorRotation.h

Core/DecoratedFormFactor/IFormFactorDecorator.h

Core/Multilayer/FormFactorDWBA.cpp

Core/Multilayer/FormFactorDWBA.h

Core/Multilayer/FormFactorDWBAPol.cpp

Core/Multilayer/FormFactorDWBAPol.h

Core/Particle/FormFactorCoreShell.cpp

Core/Particle/FormFactorCoreShell.h

Core/Particle/FormFactorCrystal.cpp

Core/Particle/FormFactorCrystal.h

Core/Particle/FormFactorWeighted.cpp

Core/Particle/FormFactorWeighted.h

Core/Scattering/IFormFactor.h

Core/Scattering/IFormFactorBorn.cpp

Core/Scattering/IFormFactorBorn.h

auto/Wrap/doxygen_core.i

auto/Wrap/libBornAgainCore.py