Function compute_parent_node 

pub fn compute_parent_node<H>(
    node: &[u8; 32],
    sibling: &[u8; 32],
    node_index: usize,
    level: usize,
) -> Result<[u8; 32], ConcurrentMerkleTreeError>
where
    H: Hasher,

Returns the hash of the parent node based on the provided node (with its node_index) and sibling (with its sibling_index).