[Core] Split sets by date

[cchallu]

Description

As requested in #579, add the possibility to split train/val/test sets by date rather than the number of timestamps or the number of windows.

Use case

No response

[marcopeix]

Very hard to have this feature stable and to validate the timestamp format. PR exists, but we don't see a robust way of doing it. See PR #1473 for more details.