Function xqsm 

pub fn xqsm(
    local_field_ppm: &[f64],
    mask: &[u8],
    grid: &Grid,
    model_onnx: &[u8],
) -> Result<Vec<f64>, OnnxError>

Run xQSM dipole inversion.

• local_field_ppm — local tissue field (ppm), column-major (nx,ny,nz).
• mask — binary brain mask (same layout); applied to the output.
• model_onnx — bytes of the exported xqsm.onnx.

Returns susceptibility (ppm), masked, in the same layout.