mv a few files, much better decoupling

Core/Detector/IDetector2D.cpp

Core/Detector/IDetector2D.h

Core/Detector/RectangularDetector.cpp

Core/Export/PyGenVisitor.h

Core/FormFactors/FormFactors.h

Core/Reflections/FormFactorDWBA.cpp

Core/Reflections/FormFactorDWBA.h

Core/Reflections/FormFactorDWBAPol.cpp

Core/Reflections/FormFactorDWBAPol.h

Core/Reflections/FormFactorTools.cpp

Core/Reflections/FormFactorTools.h

Core/Simulation/Instrument.h