Make Parallel Join Spec Compliant

[sombrek]

Proposed Changes

Closes #130

Parallel gateways now remember which sequence flows already provided a token.
The join activates every time at least one token was provided on all incoming flows. One token per flow is consumed on join.

@barmac
Exiting a sequence flow now provides a source where a token came from. That idea was taken from #143, but this PR does not cover inclusive joins. If the solution is accepted, it could be a starting point for inclusive gateways.

Visual demo

[nikku]

@sombrek This looks great! I'll check if there is a better way to store incoming tokens than global state tomorrow. Potentially these could be stored in the parent scope.

[nikku]

Thanks again for your contribution @sombrek! I was stumbling over the global state you introduced and thought there would be a simpler way, given that we already collect execution via scopes, and hence track "active executions for a given element".

The only missing bit was to apply your filtering logic to the existing scopes scopes on this node, and properly tag initiators when entering a scope (from a sequence flow exit).

Cf. ecfaba1 greatly simplifying your implementation and 59c8511 refactoring the behavior just a bit.

[nikku]

CC @barmac because the "source" is gone (again). We track such information via scopes and their hierarchy.

[nikku]

Released via https://github.com/bpmn-io/bpmn-js-token-simulation/releases/tag/v0.34.5.

[sombrek]

I was stumbling over the global state you introduced and thought there would be a simpler way, given that we already collect execution via scopes, and hence track "active executions for a given element".

@nikku Thank you for all the additional work you've put in. I reckon I could have done it without the global state, but I failed to properly translate scopes to my understanding of tokens. I'll study your changes to get a better understanding.