Function homogeneity_masks 

pub fn homogeneity_masks(
    local_field: &[f64],
    mask: &[u8],
    grid: &Grid,
    params: &HeidiParams,
) -> HomogeneityMasks

Build the three per-direction homogeneity weights from the field map.

Public so callers can inspect or reuse the weighting; heidi calls it itself.