public static function fromPolar($length, $ap, $av)
  {
    return new static($length * cos($ap) * cos($av), $length * sin($ap) * cos($av), $length * sin($av));
  }
public static function fromPolar($length, $ap, $av)
  {
    return new static($length * cos($ap) * cos($av), $length * sin($ap) * cos($av), $length * sin($av));
  }
public static function fromPolar ( $ length , $ ap , $ av ) { return new static ( $ length * cos ( $ ap ) * cos ( $ av ) , $ length * sin ( $ ap ) * cos ( $ av ) , $ length * sin ( $ av ) ) ; }
Construct coordinate by the vector length and direction.

@param $length the vector length
@param $ap angle in XY plane (in radians)
@param $av angle off XY plane (in radians)
Construct coordinate by the vector length and direction .