Check that population size is large enough to breed

If user sets pop_size = 1, the default_propagator should leave out crossover, and warn.
Or it should check that the configuration is valid and crash immediately before starting evaluating candidates.
It should be communicated more clearly what pop_size does in the tutorials and the documentation, since it has two functions (size of the starting population that is generated by using the given init propagator and size of the population used for breeding throughout the run) that should maybe be split up.