def prepare_bootstrap(estimators)
      h_est = estimators
      h_est = [h_est] unless h_est.is_a?(Array) || h_est.is_a?(Hash)

      if h_est.is_a? Array
        h_est = h_est.map do |est|
          [est, ->(v) { Daru::Vector.new(v).send(est) }]
        end.to_h
      end
      bss = h_est.keys.map { |v| [v, []] }.to_h

      [h_est, h_est.keys, bss]
    end
For an array or hash of estimators methods, returns
an array with three elements
1.- A hash with estimators names as keys and lambdas as values
2.- An array with estimators names
3.- A Hash with estimators names as keys and empty arrays as values