Function mean_into 

pub fn mean_into<T, D>(
    a: &Array<T, D>,
    axis: Option<usize>,
    out: &mut Array<T, IxDyn>,
) -> Result<(), FerrayError>
where
    T: Element + Float + Send + Sync,
    D: Dimension,

Mean reduction writing into a pre-allocated destination.

See sum_into for the contract on out. Empty inputs return FerrayError::InvalidValue.