Function heidi_on_grid 

fn heidi_on_grid(
    local_field: &[f64],
    mask: &[u8],
    chi_init: &[f64],
    grid: &Grid,
    bdir: (f64, f64, f64),
    params: &HeidiParams,
    progress: impl FnMut(usize, usize),
) -> Vec<f64>

HEIDI’s cone fill on a grid with odd extents (upstream’s), from the seed chi_init (unmasked). Returns χ unmasked.