Changing to signed types.

This is to avoid problems that were detected by compiling and running
with clang 3.5 and the -fsanitize=integer flag.  The errors were:

/Users/laprej/ROSS/ross/gvt-mpi_allreduce.c:87:38: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'unsigned long long'
/Users/laprej/ROSS/ross/gvt-mpi_allreduce.c:87:38: runtime error: unsigned integer overflow: 17 - 20 cannot be represented in type 'unsigned long long'