hdf5_ep

[svn-r5894] Purpose:
    Bug fix/Code cleanup/New Feature

Description:
    Correct problems with writing fill-values to external storage and allocate
    the data storage at the correct times.

    Also, mostly straighten out the strange code which allocates and fills
    raw data storage for datasets.  Things are still a bit odd in that the
    fill-values for chunked datasets are written when the space is allocated,
    instead of in a separate routine, but there are two reasons for this:
    it's inefficient (especially in parallel) to iterate through all the chunks
    twice, and (more importantly) the space needed to store compressed chunks
    isn't known until we've got a buffer of compressed fill-values ready to
    write to the chunk.

    Additionally, add in the H5D_SPACE_ALLOC_INCR and H5D_SPACE_ALLOC_DEFAULT
    setting for the "space time", which incorporate the previous behavior of
    the space allocation for chunked datasets.

    The default settings for the different types of dataset storage are now
    as follows:
        Contiguous - Late
        Chunked    - Incremental
        Compact    - Early

    This checkin also incorporates a change to the behavior of external data
    storage in two ways - fill-values are _never_ written to external storage
    (under the assumption that writing fill-values is triggered by allocating
    space in an HDF5 file, and since space is not allocated in the file, the
    fill-values should not be written) and external data files are now created
    if they don't exist when data is written to them.  The fill-value will
    probably need to be revisited at some time in the future, this just seemed
    like the safer course currently.

    I think I cleaned up some compiler errors also, before getting bogged down
    in the fixes for the space allocation and fill-values.

Platforms tested:
    FreeBSD 4.6 (sleipnir) w/serial & parallel.  Will be testing on IRIX64
    6.5 (modi4) in serial & parallel shortly.