ENH: Feature Request for Ungroup Method for Grouped Data Frames
Hi, thanks for your work developing `pandas`. I'd like to request a feature to add an ungroup() method for grouped data frames. It's related to this [StackOverflow](https://stackoverflow.com/questions/20122521/is-there-an-ungroup-by-operation-opposite-to-groupby-in-pandas/66879388#66879388) where I've developed a hack to extract using the `.obj` to pull out the original data frame from the grouped data frame. 

However, it would be helpful to have a method developed, which would do the extraction to prevent users from depending on my hack. 

``` python
>>> dat_1 = df.groupby("category_2")
>>> dat_1
<pandas.core.groupby.generic.DataFrameGroupBy object at 0x7fce78b3dd00>
>>> dat_1.obj
    order_date          category_2     value
1   2011-02-01  Cross Country Race  324400.0
2   2011-03-01  Cross Country Race  142000.0
3   2011-04-01  Cross Country Race  498580.0
4   2011-05-01  Cross Country Race  220310.0
5   2011-06-01  Cross Country Race  364420.0
..         ...                 ...       ...
535 2015-08-01          Triathalon   39200.0
536 2015-09-01          Triathalon   75600.0
537 2015-10-01          Triathalon   58600.0
538 2015-11-01          Triathalon   70050.0
539 2015-12-01          Triathalon   38600.0

[531 rows x 3 columns]
```
