Replace get_default_propagator utility function by corresponding EvolutionaryPropagator class

[mcw92]

Currently, Propulate's get_default_propagator utility function returns a standard evolutionary operator, combining selection, crossover, mutation, and random initialization each with a certain probability. Instead of having this function, we should move the creation of this propagator into a corresponding class to make the instantiation of propagators consistent across all optimizer flavors in Propulate.