Get MPIHelper instance at the beginning of rod3d.cc

Otherwise the program aborts for the usual mysterious reasons.

src/rod3d.cc

@@ -27,6 +27,8 @@ using namespace Dune;
 
 int main (int argc, char *argv[]) try
 {
+    MPIHelper::instance(argc, argv);
+
     typedef std::vector<RigidBodyMotion<double,3> > SolutionType;
 
     // parse data file