USAGE:
% make a quiver plot
av=quiver(x,y,u,v);
%insert arrow scale
quiver_scale(av,u, x_scale_pos,y_scale_pos,u_scale,scale_text);
% x_pos is x-axis position of the beginning of
the arrow scale vector
% y_pos --//--
% u_scale arrow scale length in units of <u>
% scale_text [optional] displays unit of the
arow scale
% if scale_text is absent no unit is displayed