CellML support: keep track of an RDF triple's element value as a string [ci skip].

We rely a lot on an RDF triple’s element’s value as a string, so rather
than generate that value every time, we keep track of it once and for
all.
CellML support: some slight improvements to the CellmlFileRdfTriples class [ci skip].

1) We renamed CellmlFileRdfTriples:: contains() and
CellmlFileRdfTriples::recursiveContains() to
CellmlFileRdfTriples::associatedWith() and
CellmlFileRdfTriples::recursiveAssociatedWith(), respectively.
2) Some minor improvements to
CellmlFileRdfTriples::recursiveAssociatedWith().