Right in the middle of a very big cleanup/refactoring task: refactoring all the XSD code.

* [ATOM_EXT,SC] Removed to_atom/2, superseded by atomic_codes/[2,3]
* [BOOLEAN_EXT,SC] New module concentrating the support for Boolean values.
* [CODES_EXT,NEW] Added atomic_codes/[2,3] and atomic_codes_goal/3.
* [CODES_EXT,SC] Removed to_codes/2, superseded by atomic_codes/[2,3].
* [DCG_GENERIC,SC] dcg_phrase/[2,3] code simplified through use of atomic_codes/[2,3]. FAULTY!
* [PROLOG_TO_RDF,SC] Treat literal assertions as instances of datatype assertions (xsd:string).
* [ROW_EXT,SC] Relocated this module to a more logical location.
* [RDF_EXT,NEW] Added categorize_rows/[4,5].
* [RDF_EXT,NEW] Added nth[0,1]_column/[3,4].
* [RDF_STORE_TABLE,SC,DOC] Renamed this module. Added documentation.
* [XSD,XSD_CLEAN,SC] Big cleanup of all XSD code. Splitting code into parsing/generating XSD values and cleaning to/between XSD values.
* [XSD_BOOLEAN,SC] Cleanup.
* [XSD_DATE,XSD_DATETIME,XSD_DATETIME_GENERIC,XSD_DATETIME_EXT,XSD_DATETIME_SUPPORT,SC] Very thorough cleanup.
* [XSD_DATETIME,NEW] Added xsd_dateTime_compare/3.
* [XSD_DURATION,NEW] Added dateTimePlusDuration/3 and xsd_diration_equal/2, xsd_duration_compare/3. PUBLIC!
* [XSD_TEST,DEB] Started adding an automated test suite for XSD. Still a long way to go...