Nodes tree-wise bpointer/fpointers

[leonardbinet]

I encountered the following conditions:

• having to deal with lots (potentially hundreds of thousands) of nodes containing data
• having to generate multiple subtrees without impacting initial tree

The only solution I had was to create for each subtree a deepcopy, but it was very greedy and expensive in terms of memory.

This pull-request implements the ability to generate shallow copies of trees (no deep-copy of nodes), and mutate multiple shallow copies of the same tree without impacting other trees hierarchies.

I do so by assigning Nodes not a single _bpointer _fpointer, but one per shallow copy of tree.

[caesar0301]

Linked issue https://github.com/caesar0301/treelib/issues/127