Function init_output_cube 

pub fn init_output_cube(
    input_path: &Path,
    output_path: &Path,
    target_beams: &[Option<Beam>],
    mode: CubeMode,
    meta: &CubeMeta,
) -> Result<(), CubeError>

Initialise an output cube by copying the input, then updating the beam headers.

For Natural mode a BEAMS binary-table extension is appended. For Total mode only the primary BMAJ/BMIN/BPA keywords are updated.