def only_missing as_a=:vector
      if as_a == :vector
        self[*indexes(*Daru::MISSING_VALUES)]
      elsif as_a == :array
        self[*indexes(*Daru::MISSING_VALUES)].to_a
      end
    end
def only_missing as_a=:vector
      if as_a == :vector
        self[*indexes(*Daru::MISSING_VALUES)]
      elsif as_a == :array
        self[*indexes(*Daru::MISSING_VALUES)].to_a
      end
    end
Returns a Vector containing only missing data (preserves indexes).