NaN pixels not plottet correctly in plots.plot_image

[rhandberg]

There is a problem with plotting images containing NaNs with plots.plot_image. Either the NaNs are not plotted with the correct color (default black) or pixels outside the (vmin, vmax) range are mistakenly plotted with black, as if they were NaN.

This also affects the movies being generate by run_ffimovie.