When selecting OBSERVATION PERIOD as primary event criteria, allow user to define cohort start date and cohort end date within the period

[pbr6cornell]

Use case: I want to define my cohort as patients who have continuous coverage from 1Jan2010 to 31Dec2012, and use that time period as my cohort definition and as the basis for my analysis. So, I want to select all observation periods with observation period start < 1Jan2010 and observation period end > 31Dec2012, but I don't want the cohort start date to become the observation period start date, i want it set to 1Jan2010. this would support analysis requiring incidence/prevalence calculations during defined intervals of time-at-risk.