Added support for several XSD defined datatypes.

* [DCG_CARDINAL,NEW] Added non-zero variants of decimal_digit// and octal_digit//.
* [RADIX,NEW] Added decimal_to_digits/3 to go from a (decimal) weight to a list of digigts in a positional system with arbitrary base.
* [RADIX,NEW] Added digits_to_digits/4 converting between arbitrary radices.
* [XSD,DOC] Added documentation.
* [XSD,DOC] Added leap second table.
* [XSD_DATE,NEW] New module processing XSD dates.
* [XSD_DATE_TIME,NEW] New module processing XSD dateTimes.
* [XSD_DURATION,NEW] New module processing XSD durations.
* [XSD_FACETS,DOC] Documented the principle facets of the XSD defined datatypes.
* [XSD_GDAY,NEW] New module processing XSD Gregorian days.
* [XSD_G_MONTH,NEW] New module processing XSD Gregorian months.
* [XSD_G_MONTH_DAY,NEW] New module processing XSD Gregorian month-days.
* [XSD_G_YEAR,NEW] New module processing XSD Gregorian years.
* [XSD_G_YEAR_MONTH,NEW] New module processing XSD Gregorian year-months.
* [XSD_HEX_BINARY,NEW] New modules processing XSD hex-encoded binary data.
* [XSD_TIME,NEW] New modules processing XSD times.
* [XSD_YEAR_MONTH_DURATION,NEW] New modules processing XSD year-month durations.