Function mean_poisson_deviance 

pub fn mean_poisson_deviance<T>(
    reference: ArrayView2<'_, T>,
    candidate: ArrayView2<'_, T>,
    valid_mask: Option<ArrayView2<'_, bool>>,
    epsilon: f64,
) -> Result<f64, IntensityMetricError>
where
    T: ToPrimitive,

Return mean Poisson deviance, flooring candidate intensities at epsilon.