ENH: Make it possible to convert types using astype inplace
#### Is your feature request related to a problem?

No, the feature is not related to a problem.

#### Describe the solution you'd like

It should be possible to perform the `astype()` operation directly into a dataframe passing an argument `inplace` as True.

#### API breaking implications

The API should not break once the default argument should be false, and the function will keep returning the same result as before.

#### Describe alternatives you've considered

Creating a new function exclusively for doing this kind of operation.

#### Additional context

```python
df = pd.DataFrame({'col1' : [1, 2, 3], 'col2' : [1.0, 2.0, 3.0]})
df.astype('float32', inplace=True)
```
