Gladstone-Dale law rerfactive index profile

In this MR I have created a refractive index profile that uses the Gladstone-Dale law to calculate that through the density profile.

corsika/media/UniformRefractiveIndex.hpp

@@ -19,7 +19,7 @@ namespace corsika {
    * for all evaluated locations.
    */
   template <typename T>
-  class UniformRefractiveIndex final : public T {
+  class UniformRefractiveIndex : public T {
 
     double n_; ///< The constant refractive index that we use.