Change `subscriptionsFn` to take a model as an argument

[astynax]

Closes #116

[astynax]

@z0w0 you are right, subscriptions should react on model changes. I'll try to dig in this direction

[z0w0]

@astynax should be able to change stepAction in Helm.hs to change actionSmp to be the result of calling subscriptionsFn with the updatedModel.

Let me know if you'll have the time to do that, otherwise I'll merge and adjust that myself :)

[astynax]

But if you call subscriptionsFn, how this new SignalGen replace the old one inside running Elerea's graph?

[z0w0]

Hmm, true. It would generate a new graph every time. Probably not ideal.

[Ulrar]

I might be off here, but if the goal is to allow changing subscriptions at runtime, wouldn't be simpler to just allow the use of a Cmd to either delete / create a subscription, or to request the call to the subscription function (i.e. Cmd.updateSubscriptions) ? That way subscriptions would be re-calculated only when needed.