Use cohort generators cohort subset operator

[gowthamrao]

CohortGenerator has a very powerful me functionality called cohort subset . Thank you @azimov I have requested some additional functionality in Cohort generator such as
OHDSI/CohortGenerator#94

I think if the Characterization package could use cohort subset operator instead of using it's own cohort subset functionality it would make the Characterization package more powerful

[jreps]

Characterization is just doing T with O intersections. I want to keep the dependancies down so will keep the code for that in Characterization. However, I recommend using CohortGenerator to create any subset cohorts for T and O and then feeding them into Characterization.