Add `n_rate` statistic to `estimate_incidence_rate()` #324

All 838 tests pass, 12 skipped in 1m 10s

1 files ±0 83 suites ±0 1m 10s ⏱️ -1s
850 tests +1 838 ✅ +1 12 💤 ±0 0 ❌ ±0
1 828 runs +1 1 150 ✅ +1 678 💤 ±0 0 ❌ ±0

Results for commit 7db0da2. ± Comparison against earlier commit 97e1ca4.