Re: Keeping R2RML free of Direct Mapping dependency (ISSUE-25)

I see at least two usage scenarios:

1) Just take an RDB and make its contents accessible as RDF. In my opinion
the Direct Mapping satisfies this need. It is not necessary to tie R2RML to
the Direct Mapping to achieve this. (This does not address the case of
mapping a _changing_ RDB schema to RDF, but from my perspective that seems
like a distant edge case that we do not need to address directly in the 1.0
version of the specs.)

2) Craft an R2RML mapping to expose a view of an RDB as RDF. This could be
targeting a pre-defined domain ontology or crafting a domain ontology as
part of the mapping exercise. In either case, I think the R2RML mapper needs
explicit control over which RDB entities are exposed.

If the user wants a hybrid of these two models then they can generate the
Direct Mapping for an RDB and then replace parts of it with a hand-crafted
R2RML mapping.

-David