Interval: handle negative multiplication

dune/solvers/common/interval.hh

@@ -63,6 +63,11 @@ public:
     {
       data_[0] *= c;
       data_[1] *= c;
+
+      // swap if multiplication with negative number
+      if ( c < 0.0 )
+        std::swap(data_[0],data_[1]);
+
       return *this;
     }