add getComm function

Solver/linearSystemPETSc.h

@@ -79,6 +79,7 @@ class linearSystemPETSc : public linearSystem<scalar> {
   std::vector<scalar> getData();
   std::vector<int> getRowPointers();
   std::vector<int> getColumnsIndices();
+  MPI_Comm& getComm() {return _comm;}
 };
 
 class linearSystemPETScBlockDouble : public linearSystem<fullMatrix<double> > {