fix: apply mutation on children

[kkafar]

Description

Mutation operator is now being called for every child coming from crossover operator.

Linked issues

Resolves #95

Important implementation details

I was not sure whether to apply mutation on:

• current generation + new children
• whole generation after replacement
• just new children

Ultimately I went with last option: children are mutated and then replacement operator happens.

There is also consideration whether we should call mutation operator as often as mutation_rate or maybe the mutation operator should be called for every individual and mutate each gene with probability of mutation_rate. There is also option to add another parameter: mutation_probability, which would dictate probability of applying mutation operator and then mutation_rate would describe probability of modifying single gene.

In this PR I went with second option: mutation operator will be called for every individual, and mutation rate will be passed to the operator.

However in the future iterations of GA implementation, IMHO, we should go with third option ;D