Selection-Variation Emitter

[Lookatator]

This PR aims at improving the flexibility of all variational operators, and combine them with the repertoire selector we want.

More specifically:

• We introduce a SelectionVariationEmitter that separates the selector and the variation operator
• All variation operators are divided into subcategories such as Mutation and CrossOver
• We also have a VariationComposer for applying several variations sequentially
• We provide an implementation for SBX and Recombination CrossOvers
• All mutations have a mutation_rate that can be specified, and all cross-overs have a cross_over_rate. All of those are by default equal to 1.
• For all variation operators, we can specify some values for minval and maxval, to automatically clip the parameters.

The UML diagram of the proposed structure is:

TODOs left:

• Tests
• Documentation