optimization: check if user buffers in the nonblocking requests are contiguous from one another. if yes we concatenate them into one buffer and maybe can skip defining a derived data type for buf_type

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1876 c368ceb8-fd28-0410-a457-e46ee1d7cd4a