update `ard_cohens_d` function to accept multiple variables #179

All 20 tests pass, 8 skipped in 4s

1 files ±0 29 suites ±0 4s ⏱️ ±0s
28 tests ±0 20 ✅ ±0 8 💤 ±0 0 ❌ ±0
71 runs ±0 52 ✅ ±0 19 💤 ±0 0 ❌ ±0

Results for commit e7493c8. ± Comparison against earlier commit a8afc0f.