replace ncmpii_data_repack with MPI_Pack or MPI_Unpack, as all outbuf/inbuf to be packed/unpacked are contiguous. This avoid an additional buffer copy

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1613 c368ceb8-fd28-0410-a457-e46ee1d7cd4a