Removed unneeded ctor argument

This MR removes the unnecessary constructor argument of index. It is unused since the following call to MPI_Unpack uses it as an output-only argument. This also imposes an additional requirement on GlobalIndex to have a constructor GlobalIndex(int) that is not required elsewhere.

1333:    PairType index(1);
1334:    MPI_Unpack(p_in, bufferSize, position, &index, 1,
1335:               type, comm_);