added operator() to change values

cpp/Tensor2.h

@@ -35,7 +35,11 @@ namespace tensors {
 			T[j] = a(j);
 	}
 
-		double operator()(int i, int j) const {
+	const double & operator()(int i, int j) const {
+		return T[j](i);
+	}
+
+	double & operator()(int i, int j) {
 		return T[j](i);
 	}