All vessel::calculate functions are now const

src/crystallization/DFSClustering.cpp

src/crystallization/GradientVessel.cpp

src/crystallization/VectorMean.cpp

src/crystallization/VectorSum.cpp

src/mapping/SpathVessel.cpp

src/mapping/ZpathVessel.cpp

src/multicolvar/BridgedMultiColvarFunction.cpp

src/vesselbase/ActionWithVessel.cpp

src/vesselbase/ActionWithVessel.h

src/vesselbase/Between.cpp

src/vesselbase/Between.h

src/vesselbase/BridgeVessel.cpp

src/vesselbase/BridgeVessel.h

src/vesselbase/FunctionVessel.cpp

src/vesselbase/FunctionVessel.h

src/vesselbase/LessThan.cpp

src/vesselbase/LessThan.h

src/vesselbase/Max.cpp

src/vesselbase/Mean.cpp

src/vesselbase/Min.cpp