FutureWarning: Downcasting object dtype arrays

**Describe the bug**
Appeared while using `ms3 extract -M -N -X -D -C -a`:

```
ms3\bs4_parser.py:4468: FutureWarning: Downcasting object dtype arrays `
  duration_df.iloc[:, 0].fillna(0).astype(int).abs()
```

**ms3 version**

2.5.3