Implement elitism

[kolergy]

Thanks for your nice code.

Elitism is extremely simple to implement and allows to ensure that the N best elements of the population are carried to the next generation avoiding the fitness drop that can be seen when running GPlearn fit

N=1 is a good value, for very large population it can be beneficial to push it up a bit.

ref: Dejong K. (1975), An analysis of the behavior of a class of a class of genetic adaptive systems. PhD Thesis, University of Michigan.

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