Mixture

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

Closes #23 (closed).

NEWS.md

 # stressaddition (development version)
 
-* Fix documentation of `ecxsys()` and `predict_ecxsys`.
+* Added predict_mixture() for the prediction of the effects of mixtures of two toxicants.
+* Fixed documentation of `ecxsys()` and `predict_ecxsys`.
 
 # stressaddition 2.0.0