Fitting widget refactor

[rozyczko]

Description

This is the refactoring effort, the same as in #2651 but reparented to the current main.

• Polydispersity widget logic moved to PolydispersityWidget.py
• Polydispersity widget UI moved to PolydispersityWidgetUI.ui
• Magnetic widget logic moved to MagnetismWidget.py
• Magnetic widget UI moved to MagnetismWidgetUI.ui
• Moved common structures to FittingLogic.py

All issues raised by @krzywon in the original PR have been addressed.

Additionally, the files involved in the refactor were "improved" by

• adding type hints throughout
• adding style changes required by ruff (running ruff check)

This is part 1 of the refactoring, focused on the two biggest and most independent components.
Next, I will attempt to refactor out constraints to a separate module.

Review Checklist:

[if using the editor, use [x] in place of [ ] to check a box]

Documentation (check at least one)

• [ X] There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • [X ] Windows installer (GH artifact) has been tested (installed and worked)
  • [X ] MacOSX installer (GH artifact) has been tested (installed and worked)

Licencing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)

[rozyczko]

macos-latest fails due to a know issue with binary signing. #3134

[lucas-wilkins]

@rozyczko - there's conflicts to resolve