- allow for multiple visits in comparative cohort

 - add parameter to only evaluate visits during cohort start and end

 - documentaion update

 - workarounds for extremely large visit_occurrence tables

 - allow external models to be used; bug fix in getPopnPrev.sql

 - change test script

Merge branch 'develop'

 - test change

 - add documnetation

 - correct eunomia test