Function pdf_core 

pub(crate) fn pdf_core(
    field: &[f64],
    mask: &[u8],
    grid: &Grid,
    bdir: (f64, f64, f64),
    tol: f64,
    max_iter: usize,
    progress: impl FnMut(usize, usize),
) -> Vec<f64>

PDF with an explicit LSMR iteration count.

Internal entry point; the public pdf wrapper supplies the default iteration count from PdfParams.