Added facility to dump multicolvars and the corresponding atom position to an xyz file

regtest/crystallization/rt-dumpmulti/64.xyz

regtest/crystallization/rt-dumpmulti/Makefile

regtest/crystallization/rt-dumpmulti/config

...t/crystallization/rt-dumpmulti/coordination.xyz.reference

regtest/crystallization/rt-dumpmulti/plumed.dat

regtest/crystallization/rt-dumpmulti/q6.xyz.reference

regtest/crystallization/rt-dumpmulti/w6.xyz.reference

src/crystallization/OrientationSphere.cpp

src/crystallization/VectorMultiColvar.cpp

src/crystallization/VectorMultiColvar.h

src/multicolvar/AdjacencyMatrixAction.cpp

src/multicolvar/Density.cpp

src/multicolvar/DumpMultiColvar.cpp

src/multicolvar/LocalAverage.cpp

src/multicolvar/MultiColvarBase.cpp

src/multicolvar/MultiColvarBase.h

src/multicolvar/MultiColvarFunction.cpp

src/multicolvar/MultiColvarFunction.h

src/multicolvar/NumberOfLinks.cpp