hdf5

[svn-r4891] Purpose:
    Code speedup
Description:
    Chunking I/O routines are reading in an entire chunk when performing I/O
    on the chunk, even if the chunk will be too large to cache.
Solution:
    If the chunk is too large to cache, uncompressed and has been allocated
    space in the file, or if we are using the MPI-I/O VFD, perform the I/O
    directly to the chunk, instead of reading the chunk into memory, updating
    it and immediately writing the entire chunk back out.
Platforms tested:
    FreeBSD 4.5 (sleipnir) (using serial access) and IRIX64 6.5 (modi4) (using
    parallel access with MPI-I/O)