Made multicolvar density use new Grid vessel

regtest/crystallization/rt-wcsurface/plumed.dat

src/core/Action.cpp

src/core/Action.h

src/multicolvar/MultiColvarDensity.cpp

src/vesselbase/ActionWithInputVessel.cpp

src/vesselbase/GridVessel.cpp

src/vesselbase/GridVessel.h

src/vesselbase/HistogramOnGrid.cpp

src/vesselbase/HistogramOnGrid.h

src/vesselbase/PrintCube.cpp

src/vesselbase/PrintGrid.cpp