how to prevent pannel toggle event in ngb accordian
public toggleAccordian( props:NgbPanelChangeEvent ): void{
   props.nextState // true === panel is toggling to an open state 
                              // false === panel is toggling to a closed state
   props.panelId    // the ID of the panel that was clicked
   props.preventDefault(); // don't toggle the state of the selected panel
}
