DefaultNewtonVector: Allow outside access to the wrapped base vector.

Should be used sparingly, but is needed, e.g., to solve systems of
linear equations.

src/JacobianSystem.hpp

@@ -129,6 +129,9 @@ namespace Opm {
             const typename Vector::value_type&
             operator[](size_t i) const { return v_[i]; }
 
+            Vector&       wrappedVector()       { return v_; }
+            const Vector& wrappedVector() const { return v_; }
+
         private:
             Vector v_;
         };