Use thermal conductivity on medium scale in HT/THM and TRM process

• adds the saturation to the EffectiveThermalConductivityPorosityMixing property
• changes access in local assembler of HT/THM and TRM to the medium propterty
• adds the medium proerty to benchmark files
• script for transferring project files can be found here: https://github.com/joergbuchwald/mini-projects/blob/master/tools/addEffectiveThermalConductivityPorosityMixing.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?