to do: 
- Write new plot function for DRMod and MCPMod
  - based on lattice graphics
  - Allow to plot raw data for plot.fitMod (for type = "normal" and
    addCovars = ~1)
- include simulator function: planMod
- can only use one linear model in Mods (what if we have two different
  max effects?)!
- allow for dose finding with active control  
- function to interactively choose guesstimates (use the slider
  function from the TeachingDemos package and wrap the plotModels
  function in it). Input: up to 9 model names, then create (up to 9)
  sliders?


