Implement Geometric Semantic Genetic Programming (GSGP)

[bgalvao]

implemented, but without reconstrcution ability.

I am planning on changing the enum:

enum Variation {
    Standard,
    GeometricSemantic(f32, bool)
}

to:

enum Variation {
    Standard,
    GeometricSemantic(f32, bool, GSHistory)
}

where GSHistory is a struct including Hashtables of references to individuals in the population.