Update description and news

DESCRIPTION

 Package: stressaddition
 Type: Package
 Title: Modeling Tri-Phasic Concentration-Response Relationships
-Version: 2.2.1.9000
-Date: 2020-03-03
+Version: 2.3.0
+Date: 2020-03-11
 Authors@R: c(person("Sebastian", 
                     "Henz", 
                     role = c("aut", "cre"), 

NEWS.md

-# stressaddition (development version)
+# stressaddition 2.3.0
+
+* `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.
 
 # stressaddition 2.2.1