Remove last occurences of IDiscreteFunction

src/language/modules/SchemeModule.cpp

src/language/modules/SchemeModule.hpp

src/output/WriterBase.cpp

src/scheme/DiscreteFunctionIntegrator.cpp

src/scheme/DiscreteFunctionP0.hpp

src/scheme/DiscreteFunctionP0Vector.hpp

src/scheme/DiscreteFunctionUtils.cpp

src/scheme/DiscreteFunctionUtils.hpp

src/scheme/IDiscreteFunction.hpp

tests/test_DiscreteFunctionUtils.cpp