interactive_evolution_timeseries should return observable of slider values

[Datseris]

I've noticed now that I want to do some advanced usage of interactive_evolution_timeseries. I want to plot additional elements into the plot(s), whose location and/or color depend on the values of the sliders.

I think it is easy to resolve this. We return an observable that contains a dictionary that maps slider names to current values. Whenever we hit the update button, this observable is updated. The user can use this observable and lift it to create their own auto-updatable plots.