Index-based version of addBackGround

Core/Computation/ConstantBackground.cpp

Core/Computation/ConstantBackground.h

Core/Computation/IBackground.cpp

Core/Computation/IBackground.h

Core/Computation/PoissonNoiseBackground.cpp

Core/Computation/PoissonNoiseBackground.h

Core/Simulation/GISASSimulation.cpp

Core/Simulation/GISASSimulation.h

Core/Simulation/OffSpecSimulation.cpp

Core/Simulation/OffSpecSimulation.h

Core/Simulation/Simulation.h

Core/Simulation/SpecularSimulation.cpp

Core/Simulation/SpecularSimulation.h