change `cohort_id` and `strata` to `subject_var` and `group_var`. #1674

All 774 tests pass, 20 skipped in 1m 13s

      1 files     81 suites 1m 13s ⏱️
  794 tests   774 ✔️   20 💤 0 ❌
1 687 runs 1 049 ✔️ 638 💤 0 ❌

Results for commit 00db6f8.