[fix][2pncmin] define thermal conductivity law using somerton method
The 2pncmin tests and model could theoretically be used with the NI model, but we do not test this at the moment, that is why we forgot to add the corresponding law to the model.hh .
This fixes the missing struct that sets the thermal conductivity law for a non-isothermal 2pncmin model. I used the somerton method for now, but this can be changed if anyone has a strong opinion on this.
We should however come up with a 2pncminNI test to check for the correct behavior.