adding `ard_survival_survfit_diff()` function #428

All 43 tests pass, 20 skipped in 8s

1 files ±0 63 suites ±0 8s ⏱️ -1s
63 tests ±0 43 ✅ ±0 20 💤 ±0 0 ❌ ±0
177 runs ±0 141 ✅ ±0 36 💤 ±0 0 ❌ ±0

Results for commit 7d7af8e. ± Comparison against earlier commit 96fef12.