Aligned RML-star to RDF 1.2

[dachafra]

We would probably need to change the name to RML-refied or something similar.

Important documents to take into account for the modifications:

• https://www.w3.org/TR/rdf12-primer/#section-triple-terms
• https://www.w3.org/TR/rdf12-concepts/#section-triple-terms-reification
• https://www.w3.org/TR/rdf12-turtle/#reifying-triples (and the other syntaxes)

[dachafra]

If RDF 1.2 is standardised, this spec will be incorporated in RML-core, see RML-WG charter