Add local_operator_cfg.hh code to RichardsModel doxygen module

dune/dorie/model/richards/local_operator_cfg.hh

@@ -13,6 +13,11 @@ namespace Dune {
 namespace Dorie {
 namespace Operator {
 
+/**
+ *  \addtogroup RichardsModel
+ *  \{
+ */
+
 //! Interior penalty type
 enum class RichardsDGMethod
 {
@@ -106,6 +111,11 @@ inline auto read_operator_settings(
     return std::make_tuple(upwinding, method, weights);
 }
 
+/**
+ *  \}
+ *  // group RichardsModel
+ */
+
 } // namespace Operator
 } // namespace Dorie
 } // namespace Dune