add MPL property for handling effective thermal conductivity on the medium level

In order to avoid if statements in the local assembler to look for the thermal conductivity (whether medium or phase values are given), this property is implemented on the media scale and reads in the corresponding phase properties. Are there better name suggestions? I'm not completely convinced yet.

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