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.