#196 Replace propagation interfaces by functors

[Ulf Lorenz]

What and Why

Several of the interfaces in the propagator package are too complex for simple use-cases. Looking at PropagationObserver, EigenstateObserver and PropagationFilter, each of them exposes only one function. However, we cannot simply replace the interfaces due to lifetime issues; they need to keep their state over several calls.

As a consequence, each of these interface should get an alternative std::function implementation. This could then be used for simple use-cases.

Acceptance criteria

• There is an alternative for registering a PropagationObserver that takes a functor
• There is an alternative for registering an EigenstateObserver that takes a functor
• There is an alternative for registering a PropagationFilter that takes a functor
• The simple classes have been changed to functors. For example, NormalizationFilter could be a simple function.
• There is a highlighted demo for each of the use-cases.