Fix m.Flow argument type for strictNullChecks

[filipesmedeiros]

Allow null Stocks

[scottfr]

Thank you, for completeness could you make a similar change for Model.Link and model.Transition?

[filipesmedeiros]

@scottfr I assumed those two were not possible. When can a link (or a transition) be created with null "tips"/ends?

[scottfr]

Dangling transitions can be used for various purposes in state/transition charts. (One could say there should alwayse be "entry" or "exit" states instead of allowing dangling transitions, but simulation does not do that.)

Dangling links, while not used in simulation's numerical simulation, but can be useful to communicate things qualitative/graphically. Both file formats the package has importers for -- Insight Maker's XML and ModelJSON -- support dangling links. I would like to support them for consistency.

[filipesmedeiros]

Done :)

[scottfr]

Thank you Filipe!