Enable optional inline parameters for MPL property SaturationWeightedThermalConductivity

This merge request introduces support for optional inline parameter definitions in MPL property configurations.

In particular:

• The MPL property SaturationWeightedThermalConductivity now supports defining its parameters inline in the project file, without requiring separate <parameter> definitions.
• Updated corresponding unit tests to cover the new inline parameter capability.
• Minor refactoring in the test code to allow constructing XML property definitions directly from parameter values for better maintainability.

These changes are backward-compatible: existing configurations using separate <parameter> elements remain valid.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?