Clean up how we specify default values for model parameters

[alexdewar]

Description

I started work on #420, but thought the code could use a little clean-up first, so I've opened this as a separate PR.

The way we currently define default values for model parameters (as read from model.toml) is with a couple of macros in src/model/parameters.rs, but this ends up being a bit verbose and ugly. I've cleaned it up by using a custom Default implementation for the ModelParameters struct instead, which lets us list the default values in a more natural way. The downside of this approach is that it does mean we need to define a "default" value for milestone_years, even though this parameter is not optional, then verify that a value was provided after the fact. I think it's worth it overall though.

While I was at it I changed the capacity_margin parameter to be a Dimensionless rather than an f64, to make it consistent with the other parameters. It's good to be explicit that it represents a ratio, not some physical value.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.76%. Comparing base (a10a1f4) to head (6684455).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1260      +/-   ##
==========================================
+ Coverage   89.74%   89.76%   +0.01%     
==========================================
  Files          57       57              
  Lines        8195     8210      +15     
  Branches     8195     8210      +15     
==========================================
+ Hits         7355     7370      +15     
  Misses        544      544              
  Partials      296      296              

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[Copilot]

Pull request overview

Refactors how model.toml parameter defaults are defined by moving from per-field default macros to a Default implementation on ModelParameters, and aligns capacity_margin with the codebase’s unit-typed parameters (Dimensionless) for consistency.

Changes:

• Replace macro-based per-field defaults in ModelParameters with #[serde(default)] + a custom Default impl, while validating required milestone_years post-deserialization.
• Change capacity_margin from f64 to Dimensionless in model parameters and propagate the type change into dispatch optimisation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/simulation/optimisation.rs	Updates DispatchRun/capacity variable plumbing to accept Dimensionless capacity_margin, converting to f64 only at the solver-boundary.
src/model/parameters.rs	Introduces ModelParameters: Default to centralize defaults, removes old default macros, improves the milestone_years validation message, and makes capacity_margin unit-typed.

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