SYSML21 — TimeInstantValue for SysML Clock

Currently, Parts in SysML default to using the KerML Clocks::Clock instead of the SysML domain Time::Clock that specializes the KerML Clock to use TimeInstantValue instead of NumericalValue

In order to support using Time within the expression system of SysML that includes units, TimeInstantValue is an important part of ensuring those units work out correctly. So, having just a NumericalValue (that would presumably default to the count of epoch seconds???) would become problematic for such ScalarQuantityCalculations and specifications using the "localClock" of those parts.

Time.sysml

	part def Clock :> Clocks::Clock {
		doc
		/*
		 * A Clock provides a currentTime as a TimeInstantValue that advances monotonically over its lifetime.
		 */
	
		attribute :>> currentTime : TimeInstantValue;
	}