Merge branch 'feature/language-item-values' into 'develop'

doc/userdoc.org

src/language/modules/MeshModule.cpp

src/language/modules/MeshModule.hpp

src/language/modules/SchemeModule.cpp

src/language/modules/WriterModule.cpp

src/language/modules/WriterModule.hpp

src/language/utils/CMakeLists.txt

src/language/utils/ItemValueVariantFunctionInterpoler.cpp

src/language/utils/ItemValueVariantFunctionInterpoler.hpp

src/mesh/ItemValue.hpp

src/mesh/ItemValueVariant.hpp

src/mesh/MeshData.hpp

src/mesh/SubItemValuePerItemVariant.hpp

src/output/GnuplotWriter.cpp

src/output/GnuplotWriter.hpp

src/output/GnuplotWriter1D.cpp

src/output/GnuplotWriter1D.hpp

src/output/INamedDiscreteData.hpp

src/output/IWriter.hpp

src/output/NamedDiscreteFunction.hpp