Macro impl_altreal_slice 

macro_rules! impl_altreal_slice {
    ($ty:ty) => { ... };
}

AltRealData for any container that derefs to [f64].

Distinguishes R’s NA_real_ from a regular IEEE NaN where R does: sum, min and max return NA for an NA element and NaN for a NaN element, while no_na treats both as missing (see real_slice_no_na).