Add missing Doxygen headers

Core/Beam/FootprintFactorGaussian.cpp

Core/Beam/FootprintFactorSquare.cpp

Core/Beam/IFootprintFactor.cpp

Core/Beam/IFootprintFactor.h

Core/Fitting/FitObserver.h

Core/Instrument/PyArrayImportUtils.cpp

Core/Instrument/PyArrayImportUtils.h

Core/Material/MaterialUtils.cpp

Core/Material/MaterialUtils.h

Core/Material/RefractiveMaterialImpl.cpp

Core/SimulationElement/DepthProbeElement.cpp

Core/SimulationElement/PolarizationHandler.cpp