parents_portion = 0 doesn't works

[mariobigai]

I'm trying to built a classical GA without parents_portion and elit_ratio parameters.
In documentation said it's only set both to zero, but doing it raises a AssertionError on line 279 of geneticalgorithm2.py file:
https://github.com/PasaOpasen/geneticalgorithm2/blob/8de8420e3da606534958c0a24db261861e1be50d/geneticalgorithm2/geneticalgorithm2.py#L279

[PasaOpasen]

@mariobigai

parents_portion is a part of parents which will be used for creating children, it cannot be 0

now its not possible to disable this feature

To disable directly u can comment lines 941 942 964 966 967 and start loop in 946 from 0, but it's not so enough