ST6RI-959 SysML 2.1 Ballot 3 - Abstract Syntax

[seidewitz]

This PR implements resolutions for issues that were approved on SysML 2.1 RTF Ballot 3 related to abstract syntax. These issue all had been pro-actively implemented, so changes for this PR are minimal.

Changes

• SYSML21-300 Constraint checkRequirementUsageSubrequirementSpecialization needs to exclude assumed requirements
  • Implemented since PR ST6RI-168 Concrete syntax for requirements #87, except that final RTF-approved resolution calls for excluding required constraints, too.
• SYSML21-302 Override namingFeature for a view RenderingUsage
  • Implemented since PR ST6RI-361 Update implementation for 2021-04 metamodel baseline #226. Updated in this PR to more thoroughly check for nulls.
• SYSML21-368 Abstract syntax changes from resolution of SYSML2_-159 were not made
  • Implemented since PR ST6RI-825 Send and accept action notation (SYSML2_-159) #626. No changes in this PR.