Issue #507: Add weighting support to marginal model class

[seabbs]

Description

This PR closes #507 by adding weighting support to the linelist method for the marginal model. This allows you to seamlessly use aggregate data for the marginal model. It is a component of #412.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[seabbs]

LGTM (self-review).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.50%. Comparing base (ceffa60) to head (6ddc7db).
Report is 651 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #509      +/-   ##
==========================================
- Coverage   94.09%   92.50%   -1.60%     
==========================================
  Files          15       16       +1     
  Lines         457      640     +183     
==========================================
+ Hits          430      592     +162     
- Misses         27       48      +21     

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[seabbs]

coverage is a false positive from other parts of the package that are untouched here due to the git changes to reduce the repo size (monitoring this). CI failure is #487 and so also a false positive.