Added alternative method for calculating minimum multicolvar

CHANGES/Unreleased.txt

regtest/multicolvar/rt21/derivatives.reference

regtest/multicolvar/rt21/plumed.dat

src/crystallization/DFSBasic.cpp

src/crystallization/Fccubic.cpp

src/crystallization/SimpleCubic.cpp

src/crystallization/Tetrahedral.cpp

src/multicolvar/CoordinationNumbers.cpp

src/multicolvar/Distances.cpp

src/multicolvar/InPlaneDistances.cpp

src/multicolvar/MultiColvarFilter.cpp

src/multicolvar/XDistances.cpp

src/multicolvar/XYDistances.cpp

src/vesselbase/AltMin.cpp