showresidual(F) displays the residuals for the fit F in a new figure,
ie: plots the difference between the data and the fit.
showresidual is also available from the item 'Show Fit Residuals' in
the EzyFit menu (see efmenu).
If F is not specified, plots the residual for the last fit.
YRES = showresidual(...) returns the residuals (YRES = YDATA - YFIT).
showresidual(F, H) plots the residual in the figure H.