Regtest for CENTER MASS

regtest/basic/rt16b/colvar.reference

+#! FIELDS time d
+ 0.000000 1.891849
+ 0.050000 1.893232
+ 0.100000 1.891007
+ 0.150000 1.877920
+ 0.200000 1.845063

regtest/basic/rt16b/plumed.dat

 # this is just to check if charges are read properly from mc file
 DUMPMASSCHARGE FILE=mcnew
+
+c1: CENTER MASS ATOMS=1-10
+c2: CENTER MASS ATOMS=11-20
+d:  DISTANCE ATOMS=c1,c2
+
+PRINT ARG=d FILE=colvar
+