Circularity MILP

[tsmbland]

Description

Similar to #1547, but doesn't require/allow users to manually mark feedback processes. These are detected based on which commodities end up out of order in the MILP result, with an added constraint to make sure that the system remains valid once feedback processes are removed (necessary for the second investment pass). I imagine this PR is the one we'll merge over #1547, if this two-pass approach proves useful, but that other PR is useful for testing different feedback_process configurations.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc
• Update release notes for the latest release if this PR adds a new feature or fixes a bug
  present in the previous release

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works