Separate parent & reader in Dataset / Set Class(es)

[mdr223]

From Siva's feedback on #103:

I am not a big fan of how this is implemented now. Maybe, we can fix this in a later PR. But I think we should distinguish between a parent and a reader.

A dataset should have a member reader. This constructor should take in an argument reader: str | Path | list | pd.DataFrame | DataReader and set the reader member.

Constructor of the class Set should take an argument parent which should be of type Set | None. It should be set to None here (the first dataset in a query dag).

This is cleaner and you can avoid some of the isinstance(node/source, Datareader) calls here and in your optimizer.