ENH: round(inplace=True)
#### Is your feature request related to a problem?

I frequently round numerical columns in my dataframe to two decimals using `pandas.DataFrame.round`. I would like to do this inplace, since it will make code simpler and presumable less prone to errors.

#### Describe the solution you'd like
`DataFrame.round` should get a new argument `inplace` such that if `True` the operation is done inplace. I think `DataFrame.apply` could also improve from this enhancement.

#### Example code
```python
import pandas as pd
import numpy as np

df = pd.DataFrame({"foo": [1.11, 2.22, 3.33, 4.44, 5.55],
                   "bar": [6.66, 7.77, 8.88, 9.99, np.nan]})

# e.g. dropna() works with inplace
df.dropna(inplace=True)

# but round() does not
df["foo"] = df["foo"].round(1)

# I would like a feature like:
df["foo"].round(1, inplace=True) # does not run

```
