Add argmax to GroupBy object
See SO question: http://stackoverflow.com/questions/19879790/indexing-dataframe-by-argmax-of-column-over-another-column

I was thinking something along the lines of:

```
interesting_rows = raw_data.groupby(["city"]).argmax(over='obt_dt')
```

which would pull out the row in each group that has the max value in the column referred to by the `over` keyword, 
