[Material] Add const for consistency.

MaterialLib/PorousMedium/Porosity/ConstantPorosity.h

@@ -31,7 +31,7 @@ public:
      *  @param variable    A variable with any double type value.
      *  @param temperature Temperature with any double type value.
      */
-    double getValue(const double variable, double temperature) const override
+    double getValue(const double variable, const double temperature) const override
     {
         (void)variable;
         (void)temperature;

MaterialLib/PorousMedium/Porosity/Porosity.h

@@ -33,7 +33,7 @@ public:
      *  @param temperature Temperature.
      */
     virtual double getValue(const double variable,
-                            double temperature) const = 0;
+                            const double temperature) const = 0;
 };
 
 }  // end of namespace