to do:
- simulator function
- can only use one linear model in Mods (what if we have two different
  max effects?)!
- allow for dose finding with active control  
- write planMod: function to calculate approximate sample size for dose-response
  modelling (criteria: significance of slope, significance of maximum
  contrast, probability of identifying the ED50)
- 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?


