Function save_nifti_gz 

pub fn save_nifti_gz(
    data: &[f64],
    dims: (usize, usize, usize),
    voxel_size: (f64, f64, f64),
    affine: &[f64; 16],
) -> Result<Vec<u8>, String>

Save data as gzipped NIfTI bytes (.nii.gz)