1. Added UTC correction on local time.
2. Added date and time combinations.

* [ISO8601,NEW] Added methods for type-checking.
* [ISO8601_DCG,NEW] Added DCG rules for combinations of dates (3 variants) and times.
* [ISO8601_DCG,FIX] Centuries were not generated in the right order: the first generation for =CC= was =|[C,C,0,0]|=, but the unit tests / standard examples expect that the shorted string, i.e. =|[C,C]|= is generated.
* [ISO8601_DCG,EXT] Added UTC correct for local times (hours and minutes).
* [ISO8601_DCG,EXT] Added a grammar feature for including/excluding the time designator.
* [ISO8601_DCG,EXT] Added a grammar feature for including/excluding the UTC correction designator.
* [ISO8601_DCG,SC] Added type-checks (introduced in module ISO8601) to all the low-level date and time component DCG rules. This speeds up parsing.
* [ISO8601_TEST,SC] New module that contains the tests of the ISO 8601 DCG.
* [ISO8601,FIX] Fixes to the type-checking predicates for hour-in-day, minute-in-hour, and second-in-minute.
* [ISO8601_DCG,FIX] Uninstantiated sign, hour, and minute for the UTC correction would not work.
minor (decomment)