ENH: A cheap copy on a dataframe
With the [deprecation](https://github.com/pandas-dev/pandas/issues/18801), there is no cheap way tell pandas that the user knows that they are modifying a copied dataframe. Libraries that use pandas cannot afford to always `copy()` after sub-setting. My [comment](https://github.com/pandas-dev/pandas/issues/18801#issuecomment-354628523) in the PR thread gives a specific example.

If resurrecting `is_copy` is not an option, then a better solution could be a method that can be applied inline.

```python
df = pd.DataFrame({'A':[1,2,3]})
df2 = df[df.A>2].as_copy()               # New method, as_copy
df2['B'] = 2
```