Continue cleaning & reorganizing of checkpoint/resume code

src/language/modules/CoreModule.cpp

src/language/modules/MeshModule.cpp

src/language/modules/SchemeModule.cpp

src/language/modules/WriterModule.cpp

src/utils/checkpointing/CMakeLists.txt

src/utils/checkpointing/Checkpoint.cpp

src/utils/checkpointing/ReadArray.hpp

src/utils/checkpointing/ReadDiscreteFunctionVariant.cpp

src/utils/checkpointing/ReadDiscreteFunctionVariant.hpp

src/utils/checkpointing/ReadIBoundaryDescriptor.hpp

src/utils/checkpointing/ReadIDiscreteFunctionDescriptor.cpp

src/utils/checkpointing/ReadIDiscreteFunctionDescriptor.hpp

src/utils/checkpointing/ReadIInterfaceDescriptor.cpp

src/utils/checkpointing/ReadIInterfaceDescriptor.hpp

src/utils/checkpointing/ReadINamedDiscreteData.cpp

src/utils/checkpointing/ReadINamedDiscreteData.hpp

src/utils/checkpointing/ReadIQuadratureDescriptor.cpp

src/utils/checkpointing/ReadIQuadratureDescriptor.hpp

src/utils/checkpointing/ReadIWriter.cpp

src/utils/checkpointing/ReadIWriter.hpp