Interpolate linearly between start and end

def value(self, progress_indicator):
        """ Interpolate linearly between start and end """
        if progress_indicator <= self.end_of_interpolation:
            return interpolate.interpolate_linear_single(
                self.initial_value, self.final_value, progress_indicator/self.end_of_interpolation
            )
        else:
            return self.final_value