Simplified the conversion from a GraphViz compound term and SVG DOM.

* [GRAPH_EXPORT,GRAPH_WEB,SC] Removed schema_web/2, export_schema/3 and write_schema/4. Part of a mistaken experiment some time ago...
* [GRAPH_EXPORT,SC] Renamed write_[graph,vertex,triples]/4 to [graph,vertex,triples]_to_dom/3.
* [GRAPHVIZ,EXT] Rewrote the writing of GraphViz DOM to file and to SVG DOM.
* [OS_EXT,FIX] Fixed open_pdf_unix/1 which did not pass the shell exit code correctly.
* [RDF_DATATYPE,SC] Added rdf_datatype/3 to the public predicates.
* [RDF_DATATYPE,NEW] Added rdf_datatype/2 for linking datatype names to datatypes URIs.
* [RDF_READ,O] Made rdf_datatype/5 more efficient by pairing datatype names to datatype URIs (see module RDF_DATATYPE).
* [SVG,SC] Added stream_to_svg/2 to the public predicates.