Re: ACTION-122: Proposal for ISSUE-18

Although I hate to make R2RML more verbose... I think I agree with
Alexander's proposal.

Furthermore I thought a bit about following Alexander's proposal but also
allowing a short-cut of just putting a table name in the TriplesMap (like it
is now)... but I don't think this is a good idea because it complicates the
mapping language. I think it is better to have a single, well considered way
of referencing a table.

-David