Some structure to riemann.

AND a hotfix to Post/PViewDataGModelIO.cpp which was saving NodeData with
negative MVertex indices in .msh files.

Post/PViewDataGModelIO.cpp

@@ -174,6 +174,7 @@ bool PViewDataGModel::writeMSH(std::string fileName, bool binary)
               return false;
             }
             int num = v->getIndex();
+            if(num < 0) num = 0;
             if(binary){
               fwrite(&num, sizeof(int), 1, fp);
               fwrite(_steps[step]->getData(i), sizeof(double), numComp, fp);