Better explanation of predict_mixture()

More details on how this works. And add a reference to the new paper in the function documentation and in stressaddition-package.R.

added documentation label

changed title from Better explanation of predict_micture() to Better explanation of predict_mixture()

changed milestone to %3.0

added in progress label

mentioned in commit edb67f95

Adding the reference is tracked in #26. Closing this because I don't have time to explain it better than that. I may revisit this in the future.

mentioned in merge request !30 (merged)

removed in progress label

closed via merge request !30 (merged)

mentioned in commit 57633ff6