Odd error message, concerning numpy using an alias versus numpy package name
This produces a poor warning

```python
>>> import pandas as pd
>>> import numpy as np
>>> np.ptp(pd.Series([1, 2, 3]))
/lib/python3.7/site-packages/numpy/core/fromnumeric.py:2495: FutureWarning: Method .ptp is deprecated and will be removed in a future version. Use numpy.ptp instead.
```

because `np.ptp` calls `Series.ptp`, which is deprecated. We should instruct the user
to convert the Series to an array before calling `ptp`.

```python
np.ptp(pd.Series([1, 2, 3]).array)
```

The warning is defined at https://github.com/pandas-dev/pandas/blob/20e4c186191859dcde2437edacaee43d8d34dc46/pandas/core/generic.py#L10197-L10199

---


In this case I am using numpy ptp, however the error messages states I am not

```
>>> dfagg = df.groupby(['idf']).agg({ 'f1' : [np.min, np.max, np.ptp] })
C:\test\lib\numpy\core\fromnumeric.py:2495: FutureWarning: Method .ptp is deprecated and will be removed in a future version. Use numpy.ptp instead.
```

NO warning
`>>> dfagg = df.groupby(['idf']).agg({ 'f1' : [np.min, np.max, numpy.ptp] })`
