Add ops to pd.DataFrame.GroupBy

# Brief Description

Add basic ops to `pd.DataFrame.groupby`, abstracting the temporary column creation before applying an aggregation. So we can do something like: 
```
grp.A.mul(grp.B, name='C').C.sum()
```


# Example API

```python
# create groupby object
grp = df.groupby('A')

# basic ops
grp['A'].mul(grp['B'],names=['C'])
```
