FutureWarning: pandas indexing

```
FutureWarning: The behavior of `series[i:j]` with an integer-dtype index is deprecated. In a future version, this will be treated as *label-based* indexing, consistent with e.g. `series[i]` lookups. To retain the old behavior, use `series.iloc[i:j]`. To get the future behavior, use `series.loc[i:j]`.
self.Z[row][i0 : i1 + 1] = _exchanges[i0 : i1 + 1]
```

Start from the indexing expression shown in the warning, `self.Z[row][i0 : i1 + 1]`, and locate its containing source file. Inspect the related `_exchanges` and `self.Z` data handling, then run the relevant project tests or checks. Done means the deprecated pandas indexing warning no longer appears while the existing behavior is preserved.

Scritto dal modello di indicizzazione a partire dal testo della issue.