Implement mean for Series[datetime64[ns]] and DatetimeIndex

Pandas 0.25.0 adds Series[datetime64[ns, tz]].mean() and DatetimeIndex.mean(). We should as well


```python

In [1]: import pandas as pd

In [2]: pd.Series(pd.date_range('2000', periods=4)).mean()
Out[2]: Timestamp('2000-01-02 12:00:00')

In [3]: pd.__version__
Out[3]: '0.25.0.dev0+862.g02b552d28'

```