Update baltic to version 0.3.0

- Added extensive docstrings to functions
- Reworked `subtree` function to be more reliable
- Streamlined `singleType` and `fixHangingNodes` functions
- Altered `traverse_tree` and `getExternal` functions to account for leaf-like objects (e.g. reticulations and clades)
- Other minor changes (variable names, kwargs)

Author: evogytis