Function rts_unpadded 

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

rts on the already padded grid.