Vignette + function for ERP sample size calculation

[lwuthrich]

• Including a vignette explaining the ERP criterion to take into account the precision of the estimated dose-response curve during sample size calculation
• Creating a function "simERP" that uses parametric bootstrapping to calculate ERP based on candidate models and sample sizes (+ summary and plot to see the results)
• Modifying the plotMods function: adding optional arguments to show EDps in the plots