Overhaul Propagators

[oskar-taubert]

The original GA propagators were designed overly generic.
There should be a GA propagator, that contains selection, crossover, and mutation, and all the base ones we no longer need can go away.

The RNG should be a required argument for the propagators that need one and should always be a numpy RNG, rather than random.
In particular: Clean up e.g. Gaussian.