ENH: when using groupby with levels of a multi-index, add option to drop levels in index of created groups
#### Is your feature request related to a problem?

I often use Groupby combined with the "level" parameter, to create groups for particular subsets of my multi-indexed df.
Yet, Those levels are again included in the created groups. I then have to drop them "manually" at the level of each group (as I will not need those, especially since they take a unique value at the level of a particular group). 

#### Describe the solution you'd like

Would it be possible to add an option to pandas.DataFrame.Groupby API, to automatically remove grouping levels from the index of created groups?


Example of my current code:
```python
df = df.groupby(level=['contract','subcontract']).apply(
    lambda grp: grp.droplevel(['contract','subcontract']).interpolate('slinear'))

```
