code: public DynamicReportBuilder addGlobalVariable(String name, AbstractColumn col, DJCalculation op) {
          globalVariablesGroup.addVariable(new DJGroupVariableDef(name, col, op));
          return this;
      }
docs: For variable registration only (can bee later referenced in custom
  expression)
  
  @param name
  @param col
  @param op
  @return