Default `Population` and `MuPlusLambda` objects in `evolve` are persistent

[jakobj]

The current evolve signature

def evolve(
    ...
    pop: Population = Population(),
    ea: MuPlusLambda = MuPlusLambda(),
    ...
) -> Population:

creates default Population and MuPlusLambda objects if none are passed to the function. Unfortunately, these will be persistent, i.e., subsequent calls to evolve seem to reuse the same objects.

To fix this the default argument should be None and they should initialized to the defaults in the body of the function if None is detected.