Polar channels

[rozyczko]

Polarized Experiment and Channel Support:

• Added properties and functions to the mock backend (Analysis.qml, Experiment.qml, Plotting.qml) for handling polarized experiments, including per-experiment polarization flags, channel counts, and methods for suggesting and loading polarized channels.
• In the Python backend, introduced logic to handle polarized experiment data: new properties for polarization flags and channel counts, methods to flatten or expand per-channel data, and updated experiment data retrieval to support per-channel display and selection.

Magnetism and Calculation Engine Handling:

• Added calculation engine properties and signals to the mock sample backend, including support for engines that can or cannot model magnetism, and methods for switching engines or enabling magnetism.
• Implemented per-layer magnetism properties and setters in the mock backend for editing magnetic SLD and moment angle.
• In the Python backend, improved error handling when switching calculation engines that do not support the sample's magnetism, emitting user-facing rejection messages instead of raising exceptions.

Plotting and Visualization Enhancements:

• Added properties and functions in the mock plotting backend for managing spin channel visibility, magnetic SLD profiles, and spin asymmetry data, enabling the UI to simulate these new types of plots and controls.

API and Internal Logic Improvements:

• Updated the Python backend's experiment data retrieval to allow for both flattened and per-channel data lists, supporting new charting requirements and ensuring backward compatibility for non-channel-aware consumers.
• Introduced a utility in the Python backend to unwrap QML QJSValue objects for robust interop with QML/JS code..