datetime64[ns] dtype in dataframe

[gianpd]

When, DataTranformer's fit_transform method is called, If some columns have a datetime format, an error is raised by the sklearn\utils\validation.py method.

I fixed it, turning any datetime columns to datetime.toordinal type

[sonichi]

Thanks @gianpdomiziani. This a reasonable approach before we add preprocessing specific to datetime type. Would you like to create a PR with your fix?