pd_utils.get_group_extreme fails if the data frame index is not unique

The following is the problematic code:

```
if is_max:
        ex_vals = groups[ex_field].idxmax()
    elif is_min:
        ex_vals = groups[ex_field].idxmin()

    ex_rows = df.loc[ex_vals]
```

If the index of `df` is not unique, then the `loc` line can match multiple rows per group.