code: AtomSymbol center(double x, double y) {
          Point2D center = getAlignmentCenter();
          return translate(x - center.getX(), y - center.getY());
      }
docs: Convenience function to center an atom symbol on a specified point. The
  centering depends on the symbol alignment.
  
  @param x x-axis location
  @param y y-axis location
  @return the centered symbol (new instance)