Splitp

Trees with more than 9 taxa will be feasible now, thanks to sparse matrices and signed sums.
Use `sparse_flattening` and `sparse_subflattening` functions for more than 9 taxa.

Splits are now written as two strings in base `(m+n)` separated by `|`.
For example, `01234567|89ABCDEF`

[Squangles](https://doi.org/10.1093/sysbio/sys072) are also implemented for the purpose of inferring quartet trees.

The final release before the tree implementation in SplitP is changed to use networkx.

This release should be used to run the code used for the results in my thesis (in the 'examples' folder)