Map python computational types to e.g. XSD

Currently the python types are stored as ```computational_type``` in ```FairVariable```, with a simple ```rdfs:comment``` predicate (a placeholder for now). It cannot be an ```rdf:type``` because it is a string, and not an IRI.

We should find a clean way (surely already exists) to map these python types to a URI for that type, maybe using ```xsd```.