Finding the max value for a hierarchy level over another level
Let's say I have a DataFrame like this:

```
l1  1     2   
l2  1  13  99  89
0   1  2   2   1
```

and for each value of l1, I want to find the corresponding max value amongst the l2 values under that l1 value. Is there an easy way to do this using groupby?

so in the above example it would be 1->2 and 2->99
