Introduce strategy pointers into IFresnelMap and SpecularComputationTerm

Core/Computation/ProcessedSample.cpp

Core/Computation/SpecularComputation.cpp

Core/Computation/SpecularComputationTerm.cpp

Core/Computation/SpecularComputationTerm.h

Core/Multilayer/IFresnelMap.cpp

Core/Multilayer/IFresnelMap.h

Core/Multilayer/MatrixFresnelMap.cpp

Core/Multilayer/MatrixFresnelMap.h

Core/Multilayer/ScalarFresnelMap.cpp

Core/Multilayer/ScalarFresnelMap.h