change `cohort_id` and `strata` to `subject_var` and `group_var`. #1671

All 774 tests pass, 20 skipped in 1m 31s

      1 files ±0     81 suites ±0 1m 31s ⏱️ +19s
  794 tests ±0   774 ✔️ ±0   20 💤 ±0 0 ❌ ±0
1 687 runs ±0 1 049 ✔️ ±0 638 💤 ±0 0 ❌ ±0

Results for commit 532eed8. ± Comparison against earlier commit 23dc605.