Merge branch 'feature/item-array-in-pgs' into 'develop'

doc/userdoc.org

src/language/modules/MeshModule.cpp

src/language/modules/MeshModule.hpp

src/language/modules/WriterModule.cpp

src/language/utils/CMakeLists.txt

src/language/utils/ItemArrayVariantFunctionInterpoler.cpp

src/language/utils/ItemArrayVariantFunctionInterpoler.hpp

src/language/utils/ItemValueVariantFunctionInterpoler.cpp

src/mesh/ItemArrayVariant.hpp

src/mesh/SubItemArrayPerItemVariant.hpp

src/output/INamedDiscreteData.hpp

src/output/NamedItemArrayVariant.hpp

src/output/WriterBase.cpp

tests/CMakeLists.txt

tests/test_ItemArrayVariant.cpp

tests/test_ItemArrayVariantFunctionInterpoler.cpp

tests/test_SubItemArrayPerItemVariant.cpp