This PR implements the resolution to the following issue, as approved in SysML v2 FTF 2 Ballot 7:

• SYSML2_-158 timeslice/snapshot featuring types required to specialize or be same as types

The approach previously taken to time slice and snapshot modeling in SysML was predicated on the presumption that KerML classes classified not only lives, but also all portions of those lives (such as time slices and snapshots), meaning that any portion must have the same type as the life of which it was a portion. However, this presumption was never actually formalized in KerML, and only appeared in a single statement in subclause 9.2.4.1 (Occurrences Overview) that "[portions] must be classified the same way as the Occurrences they are portionsOf, or more specialized." And this statement was removed in the resolution to KERML-204, making it clear that portions did not have to have the same type as what they are portions of.

The removal of this presumption allows for significant simplifications in SysML for the modeling of individuals, time slices and snapshots, as given in the resolution to SYSML2_-158. (The resolution also resolves the related issue SYSML2_-150 OccurrenceUsage missing portioningFeature.)

1. Remove LifeClass from the SysML abstract syntax (this was already implemented in PR ST6RI-827 Update metamodel for 2025-02/Beta 3 baseline #629). Instead, place a zeroOrOne multiplicity on an OccurrenceDefinition with isIndividual = true.
2. Require an OccurrenceUsage with a non-null portionKind, whose owningType is an OccurrenceDefinition or OccurrenceUsage, to subset either Occurrence::timeSlices or Occurrence::snapshots, depending on the portionKind.
3. No longer support the declaration of time slices or snapshots that are not directly nested in an OccurrenceDefinition or OccurrenceUsage. (Without introducing new syntax, it is not clear how to allow such declarations without using the prior approach of portioning the declared type of the usage, which would still have the problems described in issue SYSML2_-158. And the capability for such "standalone" time slice and snapshot declarations was not considered important enough to introduce some sort of new syntax to continue to allow it.)