[MeL] MS: Store reference to the nodes, not ptr.

MeshLib/MeshSubset.h

.../BoundaryCondition/GenericNaturalBoundaryCondition-impl.h

...ondition/GenericNonuniformNaturalBoundaryCondition-impl.h

.../BoundaryCondition/NormalTractionBoundaryCondition-impl.h

ProcessLib/CalculateSurfaceFlux/CalculateSurfaceFlux.cpp

ProcessLib/HydroMechanics/HydroMechanicsProcess.cpp

ProcessLib/LIE/HydroMechanics/HydroMechanicsProcess.cpp

ProcessLib/LIE/SmallDeformation/SmallDeformationProcess.cpp

ProcessLib/PhaseField/PhaseFieldProcess.cpp

ProcessLib/Process.cpp

...anicalPhaseField/ThermoMechanicalPhaseFieldProcess-impl.h

Tests/MathLib/TestGaussLegendreIntegration.cpp

Tests/NumLib/LocalToGlobalIndexMap.cpp

Tests/NumLib/LocalToGlobalIndexMapMultiComponent.cpp

Tests/NumLib/TestComponentNorms.cpp

Tests/NumLib/TestExtrapolation.cpp

Tests/NumLib/TestMeshComponentMap.cpp

Tests/NumLib/TestSerialLinearSolver.cpp

Tests/NumLib/TestSparsityPattern.cpp