Several maintentance updates to the RDF(S) modules.

* [PRINT_EXT,FIX] Syntax error.
* [RDF_BUILD,NEW] Added remove property.
* [RDF_CONTAINER,SC] New module supporting RDF containers (Alt, Bag, Seq). Taken mostly from RDFS_READ.
* [RDF_EXPORT,FIX] Fixed 3 nondet nondets.
* [RDF_LIST,SC] Relocated RDF list assertion.
* [RDF_READ,NEW] Added property find.
* [RDF_TERM,DOC] Added documentation of URIref equality, encoding, etc.
* [RDFS_BUILD,NEW] Added remove class.
* [RDFS_READ,NEW] Added find class.
* [RDFS_VOC,EXT] Added Web variant (SVG).
* [URI_STC,DOC] Added several pointers to explore.