SYSML2_ — Time model needs additional restrictions on ISO8601 dates

The Quantities and Units Domain Library model Time.sysml contains attribute definitions for ISO 8601 date/time string encodings and structures. However, the model could be improved by adding additional constraints to ensure these definitions are used properly:

1. In Iso8601DateTimeEncoding, add a constraint to verify ISO 8601 extended string encoding.
2. In Iso8601DateTimeStructure, specify restrictions for attributes month (1 to 12), day (1 to 31), hour (0 to 23), minute (0 to 59), second (0 to 60), microsecond (0 to 999999), hourOffset (-12 to +12), and minuteOffset (-59 to +59). Consider also supporting nanosecond (0 to 999999999) instead of microsecond (as used in e.g. the java.time package) or even multiple calcs for different levels of time-of-day precision.