Add support for P0Vector functions

src/language/modules/SchemeModule.cpp

src/language/utils/EmbeddedIDiscreteFunctionOperators.cpp

src/language/utils/InterpolateItemArray.hpp

src/output/GnuplotWriter.cpp

src/output/GnuplotWriter.hpp

src/output/GnuplotWriter1D.cpp

src/output/GnuplotWriter1D.hpp

src/output/OutputNamedItemValueSet.hpp

src/output/VTKWriter.cpp

src/output/VTKWriter.hpp

src/output/WriterBase.cpp

src/output/WriterBase.hpp

src/scheme/CMakeLists.txt

src/scheme/DiscreteFunctionDescriptorP0.hpp

src/scheme/DiscreteFunctionDescriptorP0Vector.hpp

src/scheme/DiscreteFunctionP0.hpp

src/scheme/DiscreteFunctionP0Vector.hpp

src/scheme/DiscreteFunctionType.hpp

src/scheme/DiscreteFunctionVectorInterpoler.cpp

src/scheme/DiscreteFunctionVectorInterpoler.hpp