Support (R2)RML graph maps

[mchlrch]

(this is a clone of https://github.com/zazuko/xrm/issues/135):

R2RML can output triples to specific named graph defined using rr:graphMap property.

Either as template

[] rr:subjectMap [
  rr:graphMap [ rr:template "http://www.example.com/{@type}" ];
].

or constant

[] rr:subjectMap [
  rr:graphMap [ rr:constant ex:StopsGraph ];
].

I propose to add this as a language feature in the map block. Multiple should be allowed

map Subject from source {
  graph template "http://www.example.com/{0}" with Type

  graph constant "http://www.example.com/StopsGraph"

  # Also using prefixed names if possible
  graph constant ex.StopsGraph
}

Sample XRM:

output r2rml

map Employee from EMPLOYEE {
    subject template "http://example.org/employee/{0}" with EMPNO;

    graphs
        constant "http://example.org/AllEmployeeGraph" ;
        template "http://example.org/EmployeeGraph/{0}" with EMPNO ;

    types
        schema.Person
}

Declaring graphs is only supported for R2RML, RML and CARML output. It's not supported for CSVW output.

Generally allowing to use RdfClass|RdfProperty|Datatype as constant is useful and makes sense as well IMO.

# Also using prefixed names if possible
graphs
    constant ex.StopsGraph ;

[mchlrch]

The following snippets show how graphmaps can be described in XRM and how it looks in generated R2RML

Sample XRM:

output r2rml

map ThingIntoGraphmapsMapping from ThingIntoGraphmapsSource {
    subject template "http://example.org/thing/{0}" with id;

    graphs
        constant "http://example.org/graph/omnigraph";          // constant IRI as string
        template "http://example.org/graph/thing-{0}" with id;  // templated IRI with variable(s)

        constant thing.Thing;              // IRI from Class
        constant thing.color;              // IRI from Property
        constant employee.myDatatype;      // IRI from Datatype
}

logical-source ThingIntoGraphmapsSource {
    type rdb
    source "THINGS"

    referenceables
        id
}

Generated R2RML:

PREFIX rr: <http://www.w3.org/ns/r2rml#>

<#ThingIntoGraphmapsMapping> a rr:TriplesMap ;
    rr:logicalTable [ rr:tableName "THINGS" ];

    rr:subjectMap [
        rr:template "http://example.org/thing/{id}" ;
        rr:graphMap [
          rr:constant <http://example.org/graph/omnigraph>;
        ]
         ;
        rr:graphMap [
          rr:template "http://example.org/graph/thing-{id}" ;
        ]
         ;
        rr:graphMap [
          rr:constant thing:Thing;
        ]
         ;
        rr:graphMap [
          rr:constant thing:color;
        ]
         ;
        rr:graphMap [
          rr:constant employee:myDatatype;
        ]
         ;
    ]

.

[mchlrch]

Updated documentation with https://github.com/zazuko/expressive-rdf-mapper/commit/9c1e9f8235941e1682abd1dd797038311a1827a4