DoseFinding 1.3-1

• Added maFitMod function to implement fitting of dose-response models via
  model averaging (#11, thanks to Björn Bornkamp)
• Fixed the way ED dose estimation works with type = "discrete". Previously
  the maximum effect size for ED calculation was taken to be the highest effect
  among the doses provided as arguments, rather than across the observed dose
  range for the fitted model (#18)
• replaced uses of ggplot2::aes_string, which is deprecated (#14)
• added additional tests for several functions