feat: Poisson errors in post pred code

[KSkwarczynski]

Pull request description

This came up during T2K review. But confirmed with Asher, and he thinks we should use Poisson instead of sqrt(N)

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Memory and CPU Usage Summary

### Memory and CPU Usage Summary

| Metric  |     Mean    |      Max    |
|---------|-------------|-------------|
| **RAM** |   355.93 MB |   367.16 MB |
| **CPU** |    96.81 %  |    98.00 %  |

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