Function relative_l1 

pub fn relative_l1<T>(
    reference: ArrayView2<'_, Complex<T>>,
    estimate: ArrayView2<'_, Complex<T>>,
    mask: Option<ArrayView2<'_, bool>>,
    alignment: ComplexAlignment,
) -> Result<f64, ComplexMetricError>
where
    T: Float + ToPrimitive,

Return sum(|aligned_estimate - reference|) / sum(|reference|).

aligned_estimate uses the requested ComplexAlignment fitted over the selected pixels.