Expanding table specific base cohorts

[edward-burn]

As we already have measurementCohort, and are planning to add a visit one #144, we could think of expanding this pattern to also add

• conditionOccurrenceCohort
• deviceExposureCohort
• drugExposureCohort
• observationCohort
• procedureOccurrenceCohort
  (i.e. one for each of the tables supported by concept cohort).

Doing this would add an easy way to support things like making cohorts based on source concept fields etc, so that user could use any of the fields in a table for making a cohort.