bug fix exact solution

mlmc/src/problem/StochasticEllipticProblem.hpp

@@ -55,7 +55,7 @@ public:
     }
 
     VectorField Flux(const Point &x) const override {
-        return VectorField(1.0, 0.0);
+        return VectorField(-1.0, 0.0);
     }
 
     Scalar Load(const Point &x) const override {