Add checkpoint/resume for ItemValueVariant

src/language/modules/MeshModule.cpp

src/language/modules/WriterModule.cpp

src/utils/checkpointing/CheckpointUtils.cpp

src/utils/checkpointing/CheckpointUtils.hpp

src/utils/checkpointing/INamedDiscreteDataHF.hpp

src/utils/checkpointing/ResumeUtils.cpp

src/utils/checkpointing/ResumeUtils.hpp