Adding entity index to the grid entities.

src/mesh/grids/tnlGrid2D.h

src/mesh/grids/tnlGrid2D_impl.h

src/mesh/grids/tnlGridEntity.h

src/mesh/grids/tnlGridEntity_impl.h

src/mesh/grids/tnlNeighbourGridEntityGetter1D_impl.h

src/mesh/grids/tnlNeighbourGridEntityGetter2D_impl.h

src/mesh/grids/tnlNeighbourGridEntityGetter3D_impl.h

src/mesh/grids/tnlTraverser_Grid1D_impl.h

src/mesh/grids/tnlTraverser_Grid2D_impl.h

src/mesh/grids/tnlTraverser_Grid3D_impl.h

src/operators/diffusion/tnlExactLinearDiffusion.h

src/operators/diffusion/tnlExactLinearDiffusion_impl.h

src/operators/diffusion/tnlLinearDiffusion_impl.h

src/operators/tnlAnalyticNeumannBoundaryConditions_impl.h

src/operators/tnlFiniteDifferences_impl.h

src/operators/tnlNeumannBoundaryConditions_impl.h

tests/benchmarks/share/tnl-run-heat-equation-benchmark

tests/unit-tests/mesh/tnlGrid2DTester.h