Add ARD function for `survfit` objects #43

All 11 tests pass, 8 skipped in 2s

1 files ±0 19 suites ±0 2s ⏱️ ±0s
19 tests ±0 11 ✅ ±0 8 💤 ±0 0 ❌ ±0
37 runs ±0 24 ✅ ±0 13 💤 ±0 0 ❌ ±0

Results for commit c7c2f0e. ± Comparison against earlier commit 9e69179.