Functions to compute URIs

At Semtech this week I have talked to two separate people who were
interested in R2RML and who thought it was important for R2RML to support
the conversion of data in the relational database to URIs. I believe they
were talking about both:

* the case where the data in the database contains the data necessary to
produce the URI (e.g. "John Smith" -> "http://people/john_smith"). There was
a desire expressed to do this without writing the SQL or SPARQL expression
to perform the conversion.

* the case where the data in the database does not provide sufficient data
to produce the URI (e.g. a code of "js25" needs to be translated to "
http://people/john_smith").

Of course R2RML has the template feature which can partially be used to
address the first case (but not without writing a SQL/SPARQL expression to
do case conversion on the input data), but not the second.

In the past I believe we have talked about satisfying this type of need (in
the context of ISSUE-7), by defining a framework for plugging in user
provided conversion functions without specifying the detailed form which
those plugins would take.

-David