addObservationPeriodId() is a new function that adds the number of
observation period that an observation is in.
Add density estimate to summariseResult()
addCohortName overwrites if there already exists a cohort_name column
#680 and #682.
Correct nan and Inf for missing values #674
addConceptIntersect now includes records with missing end date under the assumption that end date is equal to start date.
add* functions have a new argument called name to decide if we want a
resultant temp table (name = NULL) or have a permanent table with a certain
name. Additional functions are provided, e.g. addDemographicsQuery, where the
result is not computed.