def seed(all_individuals, individuals_sample, ids_fenotypes)
      super
      @structure = build_node individuals_sample, Nimbus::LossFunctions.average(individuals_sample, @id_to_fenotype)
    end
def seed(all_individuals, individuals_sample, ids_fenotypes)
      super
      @structure = build_node individuals_sample, Nimbus::LossFunctions.average(individuals_sample, @id_to_fenotype)
    end
Creates the structure of the tree, as a hash of SNP splits and values.

 It just initializes the needed variables and then defines the first node of the tree.
 The rest of the structure of the tree is computed recursively building every node calling +build_node+.