Tidy up. Put implementation in detail namespace.

Operators/BwdTrans/BwdTransImpl.cpp

 #include "BwdTransSumFac.hpp"
 #include "BwdTransMatFree.hpp"
 
-namespace Nektar::Operators
+namespace Nektar::Operators::detail
 {
 
+// Add different BwdTrans implementations to the factory.
 template <>
 std::string OperatorBwdTransImpl<double, ImplMatFree>::className =
     GetOperatorFactory<double>().RegisterCreatorFunction(

Operators/BwdTrans/BwdTransMatFree.hpp

 #include "Operators/OperatorBwdTrans.hpp"
 
-namespace Nektar::Operators
+namespace Nektar::Operators::detail
 {
 
 // Matrix-free implementation

Operators/BwdTrans/BwdTransSumFac.hpp

 #include "Operators/OperatorBwdTrans.hpp"
 
-namespace Nektar::Operators
+namespace Nektar::Operators::detail
 {
     
 // sum-factorisation implementation

Operators/OperatorBwdTrans.hpp

@@ -6,27 +6,6 @@
 namespace Nektar::Operators
 {
 
-// Fwd declaration
-template <typename TData>
-class OperatorBwdTrans;
-
-template< typename TData>
-struct BwdTrans;
-
-// Descriptor / traits class for BwdTrans
-template<typename TData = default_fp_type>
-struct BwdTrans {
-    using class_name = OperatorBwdTrans<TData>;
-    static const std::string key;
-    static const std::string default_impl;
-
-    static std::unique_ptr<class_name> create(std::string pKey = "")
-    {
-        return Operator<TData>::template create<BwdTrans>(pKey);
-    }
-};
-
-
 // BwdTrans base class
 // Defines the apply operator to enforce apply parameter types
 template <typename TData>
@@ -42,10 +21,24 @@ public:
     }
 };
 
+// Descriptor / traits class for BwdTrans
+template<typename TData = default_fp_type>
+struct BwdTrans {
+    using class_name = OperatorBwdTrans<TData>;
+    static const std::string key;
+    static const std::string default_impl;
+
+    static std::unique_ptr<class_name> create(std::string pKey = "")
+    {
+        return Operator<TData>::template create<BwdTrans>(pKey);
+    }
+};
+
+namespace detail {
 // Template for BwdTrans implementations
 template <typename TData, typename Op>
 class OperatorBwdTransImpl;
-
+}
 
 
 }