Improve predict_mixture

description

Multiple changes to predict_mixture.

• The concentration vectors are checked for NAs, which raise an error when found.
• The use of drc::ED() was replaced with a custom inverse weibull function, which does the same but better.
• predict_mixture() accepts multiple values for the concentration of the second toxicant. Both concentration vectors must be the same length.
• predict_mixture() returns a data frame with the concentrations and effects. Previously it was only a vector of effects.
• predict_mixture() received a new argument "effect_max" which scales the returned effect values.
• Renamed the arguments of predict_mixture() to use underscore letters a and b instad of 1 and 2. For example model_1 is now model_a.

system details

• RStudio Version: Version 1.2.5033
• R Version: 3.6.3
• OS Version: Windows 10 x64 (build 18363)

checklist

• This merge introduces new features.
  • new features are documented
  • new features have tests
• update documentation including examples (if necessary)
• update and polish NEWS.md
• update version in DESCRIPTION
• update date in DESCRIPTION
• no remaining TODO, FIXME, or debug prints anywhere in the source files
• devtools::document()
• devtools::check() without errors or warnings