2D Fit Functionality for HH

[benjaminls]

This draft PR introduces 2D fit functionality from current HH analyses. It adds complementary fitting code to run parallel with the standard mgg pipeline. At the time it was first submitted, this PR is not complete, but remains in draft state to allow feedback from the FinalFit devs prior to merge and continued migration of the 2D code changes from a separate analysis-specific fork in Gitlab.

A few notes

• We use "mjj" as the provisional label for the new fitting axis throughout the code, but we may reconsider this decision in favor of a more analysis-agnostic name.
• Please see this presentation given to the Higgs to Gamma Gamma meeting on April 28, 2025 for demonstration of the code and additional information.
• 1D Mgg and Mjj workspaces are collected and combined into a workspace containing a 2D PDF using CombineModels.py
• CombineModels.py accepts models.json files that hold 1D model information and paths to be collected for the 2D models
• After each 1D model is fit, the model name and workspace file path are written to models.json

Changes

• Added CollectModels.py 2D workspace writer
• Added models.json writer
• Added "Mgg" and "Mjj" text to output file names (workspaces, plots, json, etc.) and ROOT objects to disambiguate mgg/mjj and avoid overwrite-issues.
• Added control over mjj fitting function (commit pending)
• Changed initialization for mjj NGaussian fTest for distributions with wider waist
• Added separate control over high/low fitting and plotting ranges for mgg and mjj
• Added mjj and 2D specific object names (commonObjects.py)

As commits are added to this PR, we will update the status. We would be grateful to have any feedback from the FinalFit developers to ensure the current code aligns with their vision for the flashggFinalFit package. If the changes are considered too disruptive for the main branch, we may propose adding a designated "2D branch" instead.