Adding more testing, proper serialization, and misc. bugfixes

[andrewsalij]

This fixes a number of edge cases with group-selfies as well as introduces more comprehensive testing. Notably, GroupSELFIES indexing alphabets may be chosen (this has been changed to default to the 16-character SELFIES alphabet), though all behavior should be backwards compatible with the old index alphabet.

Within a virtual environment, it may be seen that all tests pass via

pip install .["test"]
pytest tests

The advanced.csv includes some cases that used to fail for us before fixing certain edge cases.

Apologies for the relatively large amounts of changes--we've been maintaining a separate internal fork from which this public fork includes the main changes without our CI/CD and build changes.

[andrewsalij]

These changes are compatibly-licensed (Apache 2.0) and under copyright by Triad National Security, LLC, under open source release #O5007.

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