improve control over plotting

• Give the user more control about which curves to plot. Default to the two final curves and add an argument controlling wether the toxicant and tox+env curves should be drawn. Keep the argument for the simple models. So there will be three booleans for three pairs of curves.
• Also accept the ... argument for more fine control of axes, colors, pty, etc. Remember to use hasArg().
• The axes look bad when saving the plots with type = "cairo". This comes from anti-aliasing when overlaying multiple lines.