def to_gsl
      raise NoMethodError, 'Install gsl-nmatrix for access to this functionality.' unless Daru.has_gsl?
      if dtype == :gsl
        @data.data
      else
        GSL::Vector.alloc(reject_values(*Daru::MISSING_VALUES).to_a)
      end
    end
If dtype != gsl, will convert data to GSL::Vector with to_a. Otherwise returns
the stored GSL::Vector object.