ChiralSkyrmionDensity: Make material parameters 'double'

They were 'field_type' before, which is 'adouble' when used
together with ADOL-C (the typical case).  But using 'adouble'
for parameters is unnecessary and wasteful.

dune/gfe/densities/chiralskyrmiondensity.hh

@@ -81,8 +81,8 @@ namespace Dune::GFE
     }
 
   private:
-    field_type h_;
-    field_type kappa_;
+    double h_;
+    double kappa_;
   };
 
 }  // namespace Dune::GFE