Custom __reduce__ methods to allow pickling solvers

[c-randall]

Description

Due to the C extensions, the default pickling routines pickle.dump and pickle.load did not work with the solvers. Support functions and custom __reduce__ methods were added to resolve this, allowing both CVODE and IDA to be serialized and deserialized using pickle.

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that improves speed/readability/etc.)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• No style issues: $ nox -s linter [-- format]
• Code is free of misspellings: $ nox -s codespell [-- write]
• All tests pass: $ nox -s tests
• Badges are updated: $ nox -s badges

Further checks:

• The documentation builds: $ nox -s docs.
• Code is commented, particularly in hard-to-understand areas.
• Tests are added that prove fix is effective or that feature works.