Make PreProcessor support Document objects

[ZanSara]

Problem
Right now PreProcessor (as many other "indexing" nodes) do not really support Document objects, I don't see a reason why that should be the case other than legacy. In addition, the current error message is highly misleading because, given a Document, PreProcessor will complain that the given value is "neither a list nor a Document", when it should say "neither a list nor a dictionary".

Solution
I propose to update PreProcessor to be able to deal with proper Document objects and review other nodes with document: Dict or documents: List[Dict] in their signatures.

[ZanSara]

Should be fixed by #2342, closing