Automatically convert Date in ContinuousEncoder

[ParadaCarleton]

Right now, inputting a date into a ContinuousEncoder just drops it, but it seems more reasonable to handle by converting dates into a number of days.

[ Info: Some features cannot be replaced with `Continuous` features and will be dropped: [:Year]. 

[ablaom]

In principle this sounds like a good idea. However, we do have UnivariateTimeTypeToContinuous and I would prefer to avoid code duplication.

Perhaps we could add date_time_encoder=UnivariateTimeTypeToContinous() as a new ContinuousEncoder hyperparameter, and use that to treat columns with scitype ScientificTimeType (corresponding to any machine type <: Dates.TimeType).

What do you think?

[ParadaCarleton]

Sounds good!