added virtual destructor

mlmc/src/assemble/IStochasticAssemble.hpp

@@ -14,6 +14,8 @@ public:
         config.get("AssembleVerbose", verbose);
     }
 
+    virtual ~IStochasticAssemble() {}
+
     const char *Name() const override = 0;
 
     void Initialize(Vector &u) const override = 0;