FloorMarker emits a weak redstone signal when elevator is at selected floor.

This change, if you choose to accept it, will cause the FloorMarker block to output a fairly weak signal when the elevator arrives at a floor.  This signal is sufficient to open a door and will cease when the elevator departs (closing the door).  It would also be able to light a redstone lamp and/or trigger a noteblock, thereby signalling the arrival of the elevator.