docs: def cluster(dset,min_distance,min_cluster_size,prefix=None):
      '''clusters given ``dset`` connecting voxels ``min_distance``mm away with minimum cluster size of ``min_cluster_size``
      default prefix is ``dset`` suffixed with ``_clust%d``'''
      if prefix==None:
          prefix = nl.suffix(dset,'_clust%d' % min_cluster_size)
      return available_method('cluster')(dset,min_distance,min_cluster_size,prefix)