Bug fixing in EffectiveThermalConductivityPorosityMixing: Avoided the porosity calculation

In the existing implementation of EffectiveThermalConductivityPorosityMixing, the porosity is calculated in the member function of value, which repeats the computation performed in local assembler. Besides, the porosity property of PorosityFromMassBalance cannot be used because that it requires the historical porosity value, i.e. the value of the previous time step.

With the changes in this MR, the porosity, which has already been calculated in local assemblers, is directly passed to EffectiveThermalConductivityPorosityMixing via the variable array.

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