Remove m_cell_global_index from Connectivity

• now this information is computed when needed (load balance)
• closes #11 (closed)

src/utils/PTScotchPartitioner.cpp

@@ -90,7 +90,9 @@ PTScotchPartitioner::partition(const CRSGraph& graph)
     SCOTCH_stratExit(&scotch_strategy);
   }
 
-  MPI_Comm_free(&partitioning_comm);
+  if (partitioning_comm != MPI_COMM_NULL){
+    MPI_Comm_free(&partitioning_comm);
+  }
   MPI_Group_free(&mesh_group);
   MPI_Group_free(&world_group);