Function grid_trait::grid3::ref_fn

pub fn ref_fn<'a, I, T, F>(f: F) -> KolmoRefGrid3<'a, F, I, T> where
    I: From<Vector3<i32>>,
    T: 'a,
    F: Fn(I) -> &'a T, 

Represent a coord → &Item function as a grid.