Remove move semantics and use StdMat as default implementation for all operators

• Remove move semantics during operator construction
• Add missing operator() function for consistency
• Consistently use StdMat as default implementation for all operators

Operators/AssmbScatr/AssmbScatrImpl.cpp

@@ -7,7 +7,7 @@ namespace Nektar::Operators::detail
 template <>
 std::string OperatorAssmbScatrImpl<double, ImplStdMat>::className =
     GetOperatorFactory<double>().RegisterCreatorFunction(
-        "AssmbScatr", OperatorAssmbScatrImpl<double, ImplStdMat>::instantiate,
-        "");
+        "AssmbScatrStdMat",
+        OperatorAssmbScatrImpl<double, ImplStdMat>::instantiate, "...");
 
 } // namespace Nektar::Operators::detail