Various additions and fixes for datatype use in RDF.

* [PRINT_EXT,FIX] print_list/2 would not work on atoms. We now create a memory file and stream the list representation to it. We then extract the atom from the stream.
* [RDF_CLEAN,NEW] Added rdf_convert_datatype/6. UNFINISHED!!!
* [RDF_DATATYPE,NEW] Added rdf_convert_datatype/4 for converting values between datatypes.
* [RDF_DATATYPE,EXT] Support jpg files (besides png files) for the image datatype.
* [RDF_DATATYPE,FIX] Bug due to the order of datatype predicates (specific 'image' clause should come before generic 'variable' clause).
* [RDF_GRAPH_THEORY,EXT] rdf_edge/3 no longer restricts neighbor vertices to the same graph.
* [RDF_READ,NEW] Added rdf_member/2 and rdf_memberchk/2.
* [TYPE_CHECKING,SC] Renamed uri/1 to is_uri/1.
* [XMLS,EXT] Rewrote all XML Schema datatype conversion predicates. Added separate predicates for XML Schema datatype checking.