Add support when a CSVw object is used as `rml:source`

[vemonet]

Hi @sumutcan and @ThibaultGerrier, this pull request adds code that check if the rml:source found in the parsed RDF is an object with a url property , and extract the url property value to be used as the source path if present.

Which allows to use CSVw object as rml:source in RML mappings, e.g.:

@prefix rml: <http://semweb.mmlab.be/ns/rml#>.
@prefix csvw: <http://www.w3.org/ns/csvw#>.

:TriplesMap rdf:type rr:TriplesMap;
    rml:logicalSource [
        rml:source [
            rdf:type csvw:Table;
            csvw:url "input_data.csv";
            csvw:dialect [
                rdf:type csvw:Dialect;
                csvw:delimiter ",";
                csvw:encoding "UTF-8"
            ]
        ];
        rml:referenceFormulation ql:CSV
    ];

This solves the issue mentionned in https://github.com/semantifyit/RocketRML/issues/40 without impacting the previous behavior of the RocketRML: using a string as rml:source still work as before, as I far as I tested (npm run test pass)

Merging this pull request will allow to use RocketRML in the LDWizard (cf. https://github.com/pldn/LDWizard/issues/54)

The RML team is currently discussing on clarifying the specifications around rml:source, which currently are expected to be an URI according to the RML specs. This would be a first step in the direction of supporting objects as rml:source in RocketRML, which seems to be the direction RML is taking (note that the rmlmapper-java already supports using a CSVw Table object with csvw:url for rml:source)

Let me know if there are any changes you'll like to see in the pull requests

[ThibaultGerrier]

Hey there! Looks good to me

[mightymax]

thanks!!!

[ThibaultGerrier]

published with 2.1.2