Function correlation 

pub fn correlation<T>(
    reference: ArrayView2<'_, Complex<T>>,
    estimate: ArrayView2<'_, Complex<T>>,
    mask: Option<ArrayView2<'_, bool>>,
    alignment: ComplexAlignment,
) -> Result<Complex64, ComplexMetricError>
where
    T: Float + ToPrimitive,

Return normalized complex correlation after alignment.

The conjugation convention is sum(conj(reference) * aligned_estimate) / sqrt(sum(|reference|²) * sum(|aligned_estimate|²)). Consequently, without alignment, an estimate equal to reference * exp(iθ) has correlation exp(iθ).