API for state saving and restoring (NGWPC-8996)

[PhilMiller]

Additions

Removals

Changes

Testing

Notes

• Cf NGWPC-8989 for details about more things that may need to be handled here.

Todos

• Bmi_Multi_Formulation
• Iteration over catchment features
• x

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️