Added SPRINT collective variables

regtest/multicolvar/rt-sprint/Makefile

regtest/multicolvar/rt-sprint/colvar.reference

regtest/multicolvar/rt-sprint/config

regtest/multicolvar/rt-sprint/deriv.reference

regtest/multicolvar/rt-sprint/isomers.xyz

regtest/multicolvar/rt-sprint/plumed.dat

src/multicolvar/AdjacencyMatrixAction.cpp

src/multicolvar/AdjacencyMatrixAction.h

src/multicolvar/AdjacencyMatrixVessel.cpp

src/multicolvar/AdjacencyMatrixVessel.h

src/multicolvar/Density.cpp

src/multicolvar/MultiColvarFunction.h

src/multicolvar/Sprint.cpp

src/multicolvar/StoreCentralAtomsVessel.cpp

src/vesselbase/ActionWithVessel.h

src/vesselbase/StoreDataVessel.h