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 header, then updating the beam headers, closing the file once. The data unit is zero-filled by the close.

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

The streaming cube write path uses CubeWriter::create instead, which keeps the handle open so the data unit is written a single time. This function remains for callers that initialise then write planes through a separate handle (e.g. write_channel).