Function qsmgan 

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

Run QSMGAN on a background-removed local field (ppm), column-major (nx,ny,nz). Returns susceptibility (ppm), masked, in the same layout.