	This is part of the clusters module
	It is only available if you configure PLUMED with ./configure –enable-modules=clusters . Furthermore, this feature is still being developed so take care when using it and report any problems on the mailing list.

Calculate the number of atoms in the cluster of interest

CLUSTERS	the label of the action that does the clustering
CLUSTER	( default=1 ) which cluster would you like to look at 1 is the largest cluster, 2 is the second largest, 3 is the the third largest and so on.