SYSML21 — Sequence View should also support streaming flow relationships

In addition to messages, the sequence view should also support streaming flow relationships. Messages only specify abstract communication between entities and, for example, cannot be instantiated in simulations. Streaming flows specify concrete communication between entities, which can also be visualized very effectively in a sequence views.

Currently, sequence views only support messages and successions:
sq-graphical-relationship = message | sq-succession