SYSML21 — Flows::messages and Flows::flows subset each other

In the Systems Library model Flows, the flow usage flows explicitly subsets messages. However, messages is also a flow usage, and it has owned end features. So, according to the constraint checkFlowUsageFlowSpecialization, messages is required to subset flows. This means messages and flows subset each other and are, therefore, semantically equivalent. As a result, all message flow usages with implied subsetting of messages end up having the same semantics as non-message flow usages with implied subsetting of flows.