[rozyczko]

This pull request introduces significant improvements to the handling of multiple contrasts and experiment-model associations, as well as enhancements to fitting logic and error handling in the analysis backend. It also adds new demo datasets and updates the user interface to better support advanced fitting workflows. The most important changes are grouped below.

Multiple Contrasts and Experiment-Model Management:

• Added support for associating each experiment with a specific model, including new API methods and properties for managing these associations, tracking included experiments in fits, and updating experiment selection when experiments are removed (setModelOnExperiment, experimentsModelIndices, experimentsIncludedInFit, setExperimentIncludedInFit, follow_removed_experiments)

Fitting Logic and Error Handling Improvements:

• Refactored fitting logic to use a prepared fit object for improved thread safety and clarity, and enhanced the recording of fit results and error/cancellation handling
• Improved pre-fit error checking to handle minimizers that require finite bounds, and ensured only unique, enabled, free parameters are checked

API and Property Additions:

• Added new properties and methods for minimizer settings and fit objectives in the mock backend, preparing for expanded fitting options in the UI (fitObjectives, fitObjective, minimizerOptions, etc.)

Other Enhancements:

• Added color coding for experiments with no model and improved reporting of model names and colors for experiments.
• Extended the fit results dictionary to include more detailed fit statistics and dataset-level results.