Simplify `ParentLayer.cpp` and `SnowLayer.cpp`

[dogukanteber]

Layer class has the following virtual methods:

virtual double getFrzThermCond()=0; // get frozen layer thermal conductivity
virtual double getUnfThermCond()=0; // get unfrozen layer thermal conductivity
virtual double getFrzVolHeatCapa()=0;// get frozen layer specific heat capcity
virtual double getUnfVolHeatCapa()=0;// get unfrozen layer specific heat capacity
virtual double getMixVolHeatCapa()=0;//Yuan

The implementation of some of these methods are same in ParentLayer.cpp and SnowLayer.cpp. We cannot merge the same methods in a single method. So, this PR attempts to explain why these methods have the same implementation. It also cleans up the method implementation.