code: public static ProjectedCentroid make(long[] dims, Relation<? extends NumberVector> relation, DBIDs ids) {
      ProjectedCentroid c = new ProjectedCentroid(dims, RelationUtil.dimensionality(relation));
      for(DBIDIter iter = ids.iter(); iter.valid(); iter.advance()) {
        c.put(relation.get(iter));
      }
      return c;
    }
docs: Static Constructor from a relation.
  
  @param dims Dimensions to use (indexed with 0)
  @param relation Relation to process
  @param ids IDs to process
  @return Centroid