Normalization should cycle multiple times

[lilyminium]

Normalization currently only cycles through each reaction once. However, as implemented in RDKit it should ideally go back to the start and start again if the molecule changes, in case a later reaction has opened up an earlier one.

This would be a breaking change for our current models so must go into the next major release of NAGL. In addition, a condition should be included in GNNModel that checks the model version, compares it to the NAGL version, and warns or errors if it's being loaded with a version that has this change.

Current environment

• Which version are you using? (run python -c "import openff.nagl; print(openff.nagl.__version__)")
• Which version of Python (python -V)?
• Which operating system?
• What is the output of pip list?
• If you use conda, what is the output of conda list?