Function prc_on 

fn prc_on(
    inside: &Support,
    v: impl Fn(usize) -> f64,
    p: (f64, f64),
) -> (f64, f64)

(lo, hi), the prctiles p of v(l) over the mask’s voxels l, for the weights min(max((hi − v)/(hi − lo), 0), 1)