public function group($value = null)
    {
        if (func_num_args()) {
            $this->group = $value;
            return $this;
        }
        return $this->group;
    }
public function group($value = null)
    {
        if (func_num_args()) {
            $this->group = $value;
            return $this;
        }
        return $this->group;
    }
public function group ( $ value = null ) { if ( func_num_args ( ) ) { $ this -> group = $ value ; return $ this ; } return $ this -> group ; }
Set the group of the DataTree. Multiple DataTrees of the same
group will be able to exchange items.

@param null $value
@return $this|int
Set the group of the DataTree . Multiple DataTrees of the same group will be able to exchange items .