Resolve semantics of gaps across n-lifeline sequences

[MattWindsor91]

The semantics of a gap in a system diagram is to be a TCHAOS over the allowed set on the single module lifeline interrupted by the action of the action step. This issue is me saying that I need to think about the correct way to generalise this semantics.

Some possible approaches:

• Forbid gaps in n-lifetime sequences (seems too restrictive to me?);
• Affect only the semantics of the connected actors, with the other actors behaving as per their previous and future steps (maybe this is all we need?);
• Affect all actors, with the unconnected actors getting a TCHAOS interrupted by an internal synchronisation communication occurring after the gap actors.

It'd be nice to have the second semantics if possible, but maybe it's counterintuitive.

[MattWindsor91]

Done (we decided to make it affect all actors, but with a heavy synchronisation on both sides).