Function open_node 

pub fn open_node<L, A>(
    node_id: NodeId,
    budget: f64,
    temperature: f64,
    load_node: L,
    access_filter: A,
) -> Result<OpenResult, String>
where
    L: FnMut(NodeId) -> Result<Option<Node>, String>,
    A: FnMut(NodeId) -> bool,