function reduce($key,$handler,$finalize=null){
		$this->_reduce[$key]=[$handler,$finalize];
	}
function reduce($key,$handler,$finalize=null){
		$this->_reduce[$key]=[$handler,$finalize];
	}
function reduce ( $ key , $ handler , $ finalize = null ) { $ this -> _reduce [ $ key ] = [ $ handler , $ finalize ] ; }
Add reduce handler for grouped fields
@param $key string
@param $handler callback
@param $finalize callback
Add reduce handler for grouped fields