Properties of type rdfs:literal are not recognized as simple variables

**Description**
Converting properties with the range rdfs:literal as strings

**How to reproduce**
When importing a vocabulary using rdf:literal to define data properties, those types are not recognized.

I don't know if I am missing something but it seems like it would be great to add http://www.w3.org/2000/01/rdf-schema#Literal to the base mapping as strings?