[bugfix][Release] Initialize index with an unsigned value

With the recent changes to bigunsignedint, the index cannot
be initialized with a negative value anymore. We change this
to 1 to overcome the problem.

This commit together with the recent bugfixes by Ansgar
fixes the AMG code on master.

dune/common/parallel/remoteindices.hh

@@ -1317,7 +1317,7 @@ namespace Dune {
     if(remoteEntries==0)
       return;
 
-    PairType index(-1);
+    PairType index(1);
     MPI_Unpack(p_in, bufferSize, position, &index, 1,
                type, comm_);
     GlobalIndex oldGlobal=index.global();