Support (R2)RML graph maps

[mchlrch]

(this is a clone of #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 zazuko/expressive-rdf-mapper@9c1e9f8