[MPL] Merge thermal conductivity models / add geometric mean model

This MR merges SaturationDependentThermalConductivity and SoilThermalConductivitySomerton into one property and adds a geometric mean model.

Fixes #3318 (closed)

A script for converting old to new project files can be found here: convertOldThermalConductivitiesToNew.py

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

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