ST6RI-874 Features subset each other in Transfers and Flows library models (KERML11-78, SYSML21-324)

[seidewitz]

This PR proactively fixes the following issues, even though resolutions for them have not yet been approved by the KerML 1.1 RTF or SysML 2.1 RTF:

• KERML11-78 Transfers::transfers and Transfers::flowTransfers subset each other
• SYSML21-324 Flows::messages and Flows::flows subset each other

Model Libraries

Kernel Semantic Library

Transfers

• Made the features transfers and messageTransfers steps instead of flows. As a result, these features no longer have implied subsettings of flowTransfers.

Systems Library

Flows

• Removed the end features from the flow definition Message and the flow usage messages. As a result, messages no longer has an implied subsetting of flows.
• Changed the keyword used in the declaration of flows and successionFlows from message to flow, since these flow usages still have owned end features and, therefore, are not message flows.

Validation

KerMLValidator

• Corrected the implementation of the check of the constraint validateConnectorBinarySpecialization to properly handle redefined features when counting connector ends. Otherwise, it was incorrectly reporting a validation error on messages as modified above.

Backward Incompatibilities

1. The end features source and target of Messages and messages are now inherited and are no longer occurrence usages. This means that they cannot be used as the referenced occurrences in an event occurrence usage. However, this is appropriate, because the referenced events for a message should instead be sourceEvent and targetEvent which are still occurrence usages.
2. The correction to the validateConnectorBinarySpecialization check may result both in cases in which a previously valid model becomes invalid or a previously invalid model becomes valid.