This functions changes cohort end date to subject's death date. In the case
were this generates overlapping records in the cohort, those overlapping
entries will be merged.
IDs of the cohorts to modify. If NULL, all cohorts will be
used; otherwise, only the specified cohorts will be modified, and the
rest will remain unchanged.
requireDeath
If TRUE, subjects without a death record will be dropped,
while if FALSE their end date will be left as is.