Support mean() for Timestamps.
[Fitting a linear model to Timestamps currently does not work out of the box.](https://stackoverflow.com/questions/50979873/linear-fit-to-pandas-datetime64-values)
and pandas does not support the ols function any further. When using e.g. seaborn to make a fit it throws an error. 
> TypeError: reduction operation 'mean' not allowed for this dtype

I wonder whether the dtype can be change so that mean is supported? And whether this would make it compatible with other packages, such as scikit-learn, seaborn, etc. Also, the mean of some dates could be useful for other use cases. It does make sense to calculate the mean of some dates doesn't it? 