proportion_ca -> proportion_sa

Rename proportion_ca to proportion_sa to match the change in Indicate. And looks like it should have a completely different name? Wait for the change in Indicate and then simply copy it here.
Update the documentation for that argument.
And while you're at it rename everything in that mixture function ending in *_sam to *_sa (for "stressaddition").

Edited Apr 30, 2020 by Sebastian Henz