Capture distinction between provisional and mandatory arrows

[MattWindsor91]

Targeting the tick-tock model has pushed the issue of whether arrow actions are provisional (eg, we don't require the model to actually accept them when the environment offers them, at least not initially) or mandatory (eg. we do) onto the forefront of the sequences semantics. My feeling is that we should:

• capture both;
• choose one to be the default behaviour and provide a feature flag to enable another.

Some possible dichotomies which would appear as keywords:

• provisional / mandatory;
• hot / cold (the LSC one, which I think is the closest thing to this in the literature);
• observed / ?
• ? / urgent

I'm unsure what the graphical distinction would be, but probably the same as LSC. LSC temperature semantics might be the best initial way of representing this in the textual one, too.

Note that this dichotomy doesn't appear in other actions, as they have no initial events other than tock, and actions are defined as things that can take an arbitrary amount of time to occur (and so, they always accept tock, #65 notwithstanding).

[MattWindsor91]

We use hot/cold at the moment, but the name may change later.