[ENHANCEMENT] Add warning step when dealing with string index instead of int / datetime

[vtoliveira]

Is your proposed enhancement related to a problem? Please describe.
Sometimes you create a dataframe with date as a string format, then when doing prediction, it will raise following error:

The thing is that happens only at prediction time, and not at fitting time. Since depending on your dataset, you spend a lot of time fitting model, would be nice to inform or even try to change index format apriori.

Describe the solution you'd like
Maybe trying to infer freq at prediction time, or just a Warning advising for a pd.to_datetime transformation.

Describe alternatives you've considered
Trying to change index of nodes after fitting, but this is burdensome.