Fix non-virtual destructors in base classes

Operators/Operator.hpp

@@ -31,6 +31,7 @@ template <typename TData> OperatorFactory<TData> &GetOperatorFactory();
 template <typename TData> class Operator
 {
 public:
+    virtual ~Operator() = default;
     template <typename TDescriptor>
     static std::shared_ptr<typename TDescriptor::class_name> create(
         std::string pKey = "")

Operators/OperatorBwdTrans.hpp

@@ -11,6 +11,8 @@ namespace Nektar::Operators
 template <typename TData> class OperatorBwdTrans : public Operator<TData>
 {
 public:
+    virtual ~OperatorBwdTrans() = default;
+
     virtual void apply(Field<TData, FieldState::Coeff> &in,
                        Field<TData, FieldState::Phys> &out) = 0;
     virtual void operator()(Field<TData, FieldState::Coeff> &in,