Revise model for filters on term maps

[Aklakan]

The current internal model for RML to SPARQL translation does not handle term maps and their corresponding lateral blocks as first class citizens, such that filters on term maps can not yet be easily translated. A revision of the internal model and the translation procedure is needed:

Example: Given the following term map:

 rr:predicateObjectMap [
      rr:predicate coy:hasAssetId;
      rr:objectMap [
        rml:reference "asset_id";
        rr:datatype xsd:string ;
        norse:rml.filter "?asset_id != ''" ; # filter out empty column
      ]
    ] ;

In the RML-to-SPARQL translation, RML term maps should be be translated to their own SPARQL LATERAL blocks, as sketched below:

  logicalSource
  LATERAL {
        { FILTER(?asset_id != '') BIND(?asset_id AS ?allocatedTermMapVar) }
      UNION
        { ... }
  }

Note, that filters on the logical source just drop bindings of that source, so on that level the current model shouldn't have much problems.