Add Plant N Demand and N Limiting

[Alomir]

Summary

This PR:

• Adds...

How was this change tested?

List steps taken to test this change, with appropriate outputs if applicable

...

Reproduction steps

If appropriate, list steps to reproduce the change locally

Related issues

• Fixes # (or "Relates to #" if this is not a resolution of that ticket)

Checklist

• Related issues are listed above
• PR title has the issue number in it ("[#] <concise description of proposed change>")
• Tests added for new features
• Documentation updated (if applicable)
• docs/CHANGELOG.md updated with noteworthy changes
• Code formatted with clang-format (run git clang-format if needed)

For model structure changes:

• Removed \fraktur font formatting from docs/model-structure.md for implemented features

---

Note: See CONTRIBUTING.md for additional guidance. This repository uses automated formatting checks; if the pre-commit hook blocks your commit, run git clang-format to format staged changes.