ST6RI-733 Syntax-related issue resolutions from SysML v2 FTF Ballot #10

[seidewitz]

This PR implements resolutions of issues from SysML FTF Ballot 10 that called for updates to the SysML abstract syntax.

Resolutions of the following issues are implemented in this PR:

• SYSML2-499 Assignments parsed without a target will fail validateAssignmentActionUsageArguments
• SYSML2-616 User-defined keywords are not allowed on control nodes

Resolution of the following issue was already handled in PR #527:

• SYSML2-430 Subsetting of subjectParameter properties is wrong

Resolutions of the following issues were already implemented as approved:

• SYSML2-529 OCL for deriveViewDefinitionViewCondition and deriveViewUsageViewCondition are wrong
• SYSML2-558 checkStateUsageSpecialization constraint is incorrect
• SYSML2-560 Add note on FlowFeature direction to textual notation grammar
• SYSML2-597 checkFlowConnectionUsageSpecialization is too restrictive about FlowConnections
• SYSML2-620 Textual notation production for Comment is wrong

Resolutions of the following syntax-related issues did not affect the implementation:

• SYSML2-611 OCL rule deriveDefinitionOwnedFlow references non-existant class called FlowUsage
• SYSML2-612 OCL rule deriveUsageNestedFlow references non-existant class called FlowUsage