Study population improvements

[egillax]

• Minor speed improvements by moving filter operations before joins
• Fixed a bug where creatingStudyPopulation with an existing pop failed sometimes (I think when multiple outcome cohorts were present in plpData$outcomes and/or when each subject had multiple observations)

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (b6fb110) 89.41% compared to head (5bb4c8a) 89.38%. Report is 1 commits behind head on develop.

Files	Patch %	Lines
R/PopulationSettings.R	71.42%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #424 +/- ## =========================================== - Coverage 89.41% 89.38% -0.03% =========================================== Files 49 49 Lines 9827 9829 +2 =========================================== - Hits 8787 8786 -1 - Misses 1040 1043 +3 ```

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