Mixture

Adds predict_mixture() to predict the effects of a mixture of two toxicants.

Closes #23 (closed).

NEWS.md

 # stressaddition (development version)
 
+* Added predict_mixture() for the prediction of the effects of mixtures of two toxicants.
+
 # stressaddition 2.0.0
 
 * Changed the order of arguments in `ecxsys()`.