Bayesian_Cognitive_Modeling: extracted Stan code from R files; updated some deprecated Stan functions

[gilleslijnzaad]

This is my first pull request ever, so apologies if I did something wrong.

In many of the example models for Bayesian_Cognitive_Modeling the Stan code was defined as a string in the R file. In the README, @martin-smira was requesting that someone extract this code and create separate Stan files for them. So that's what I did.

I also changed some deprecated Stan functions, such as: <- is now =, increment_log_prob(u) is now target += u.