Function split_blocks 

pub fn split_blocks(
    blocks: &[Block],
    ncells_per_block: usize,
    direction: Option<SplitDirection>,
) -> Vec<Block>

Split blocks to achieve approximately ncells_per_block cells per block.

The split preserves the greatest common divisor of (imax-1, jmax-1, kmax-1) to maintain multi-grid compatibility.

If direction is None, the shortest axis of each block is chosen automatically (preserves Python behavior).