Improve access to item->item connectivity

src/mesh/Connectivity.cpp

src/mesh/Connectivity.hpp

src/mesh/ConnectivityComputer.cpp

src/mesh/IConnectivity.hpp

src/mesh/ItemToItemMatrix.hpp

src/mesh/MeshData.hpp

src/mesh/MeshNodeBoundary.hpp

src/mesh/SubItemValuePerItem.hpp

src/output/VTKWriter.hpp

src/scheme/AcousticSolver.hpp