Add tanh saturation function

[juanitorduz]

Could be useful to add the 2-parameter saturation function we've got outlined in this blog post https://www.pymc-labs.io/blog-posts/reducing-customer-acquisition-costs-how-we-helped-optimizing-hellofreshs-marketing-budget/ We find this works well because the parameterisation has independent control of the initial slope and the saturation level.