[ntuple] Enable subfield access in `REntry`

[enirolf]

This PR adds the possibility to add subfields (with a provided parent) to the RNTupleModel, and subsequently access their values in the REntry using their fully qualified names (parent.subfield). Checks are included to ensure that the requested subfield is an actual valid, existing subfield of the provided parent.

This functionality will be used in the RNTupleProcessor, which provides an iterator over an REntry.