Contour plot

[lindahua]

It is useful to have a contour plotting function, one that similar to MATLAB's.

[nolta]

Thanks to @jverzani, basic contour plotting now works:

f(x,y) = sin(x)*sin(y)
x = linspace(-pi, pi, 50);
y = linspace(-pi, pi, 50);
c = Winston.contourc(f, x, y);
plot(c) |> Winston.tk