Remove m_cell_global_index from Connectivity

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

src/utils/Partitioner.cpp

@@ -15,8 +15,10 @@ Partitioner::partition(const CRSGraph& graph)
   case PartitionerLibrary::ptscotch: {
     return PTScotchPartitioner::partition(graph);
   }
+    // LCOV_EXCL_START
   default: {
     throw UnexpectedError("invalid partition library");
   }
+    // LCOV_EXCL_STOP
   }
 }