Improvements to DBNL. Work in progress...

* [ATOM_EXT,NEW] Added atom_until/4 that basically performs a single expclusive split operation on an atom.
* [DBNL,EXT] Lots of extensions, still not able to scrape the DBNL: some difficulties with the URIs of the source texts.
* [RDF_DATATYPE,EXT] Added rudimentary support for the prasem:file datatype (similar to prasem:image).
* [URI_EXT,NEW] New module for URI related predicates. Currently contains:
** uri_to_file/2 for writing the contents of a URI resource to a given file.
** uri_query/3 for extracting name-value pairs from a URI's search query substring.