feat: create Rust API's for scenarios in which nodes in the Newick string are not represented numerically

[madelinegordon]

Currently, the I/O scenarios are Python-specific. As a nice-to-have, if we wanted to extend the I/O to other language scenarios, we would need corresponding API's in the Rust project for taking a Newick string from labelled format to integers, and vice-versa.

Create the following public API's in newick/mod.rs:

• read_newick_labeled, which will take in a newick string and return the modified Newick string and dictionary which maps integers to species names

• apply_newick_labels - takes in Newick string with nodes as integers as well as the mappings dictionary, applies the mappings, returns the full Newick

• Add Python API's in lib.rs