Create cohort-based standard table 1

The current createTable1() function relies on concept-based covariates. We should create an alternative table 1 based on cohorts. For example, instead of 'Diabetes type 2' being based on the occurrence of a single condition concept, it should be based on a more complex definition, possibly involving antidiabetes treatments, lab values, etc.

I propose to use the same set of items as the current table 1 (see specs here), since those were selected before to be 'good enough'. For each item (except possible demographics) we should find a corresponding cohort definition. We could then

1. load those cohort definitions into FeatureExtraction.
2. add a function in FeatureExtraction to retrieve all these cohort definitions (in a format suitable for CohortGenerator),
3. add a function for creating cohort-based covariates specifically for this set of cohorts.
4. adapt the createTable1() function for working with this new table 1.

@gowthamrao: I'd imagine the cohort definitions would come from the OHDSI Phenotype Library.

Also looping in @pbr6cornell .

Start by reviewing createTable1() and inst/csv/Table1Specs.csv to understand the existing table 1 items. Then investigate how cohort definitions from the OHDSI Phenotype Library can be loaded and returned in a CohortGenerator-compatible format. Done means cohort-based covariates can be created for the specified items and createTable1() supports the new cohort-based table.