Subset sql query builders and modifications to generate cohorts

[azimov]

This implements demographics and cohort subsetting operations including addCohortSubsetDefinition and modifications for cohort generation in manner that treats base cohorts and subsets as similar entities.

This is a fully working implementation however, what is now needed:

• More eyes to spot obvious mistakes
• Checking that the logic of all the Subset operations is correct by validating cohorts
• More examples of how to build complex queries/definitions

[codecov[bot]]

Codecov Report

Merging #73 (1f62711) into develop (2611e73) will decrease coverage by 3.44%. The diff coverage is 91.59%.

:exclamation: Current head 1f62711 differs from pull request most recent head a2f8b76. Consider uploading reports for the commit a2f8b76 to get more accurate results

@@             Coverage Diff             @@
##           develop      #73      +/-   ##
===========================================
- Coverage    99.00%   95.56%   -3.44%     
===========================================
  Files            9       12       +3     
  Lines          703     1264     +561     
===========================================
+ Hits           696     1208     +512     
- Misses           7       56      +49     

Impacted Files	Coverage Δ
R/CohortTables.R	100.00% <ø> (ø)
R/Subsets.R	87.86% <87.86%> (ø)
R/SubsetDefinitions.R	90.52% <90.52%> (ø)
R/CohortDefinitionSet.R	97.34% <96.15%> (-0.20%)	:arrow_down:
R/SubsetQueryBuilders.R	98.27% <98.27%> (ø)
R/CohortConstruction.R	98.83% <100.00%> (+0.45%)	:arrow_up:

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[anthonysena]

Thanks @schuemie for the approval - @chrisknoll and I are still doing a review/test of this functionality so we'll hold on merging this until that work is complete.

[azimov]

@schuemie i didn't mean to request a re-review, i just have fat fingers

[azimov]

@anthonysena This PR should be ready to merge into develop - the vignette has now been updated to cover most functionality