Better mayavi -> matplotlib conversion

fielddata: plot_quiver3d figsize renamed to size
        (because it's in pixels, not inch-size of the figure)
plot_quiver3d_to_2d: revamped to be more user friendly,
        directly specify dpi now,
        automatically detecs magnification to get high resolution.
plottools: Added plot_3d_to_2d as a separate function,
        that should be invoked after a mayavi plot.
        Allows fine tuning of 3D plot separate from following 2D plot.