vchoi_fork

[svn-r5834] Purpose:
    Large code cleanup/re-write

Description:
    This is phase 1 of the data I/O re-architecture, with the following changes:
        - Changed the selection drivers to not actually do any I/O, they
            only generate the sequences of offset/length pairs needed for
            the I/O (or memory access, in the case of iterating or filling
            a selection in a memory buffer)
        - Wrote more abstract I/O routines which get the sequence of offset/
            length pairs for each selection and access perform the I/O or
            memory access.

    Benefits of this change include:
        - Removed ~3400 lines of quite redundant code, with corresponding
            reduction in the size of library binary.
        - Any selection can now directly access memory when performing I/O,
            if no type conversions are required, instead of just "regular"
            hyperslab and 'all' selections, which speeds up I/O.
        - Sped up I/O for hyperslab selections which have contiguous lower
            dimensions by "flattening" them out into lesser dimensional objects
            for the I/O.

    No file format or API changes were necessary for this change.

    The next phase will be to create a "selection driver" for each type of
        selection, allowing each type of selection to directly call certain
        methods that only apply to that type of selection, instead of passing
        through dozens of functions which have switch statements to call the
        appropriate method for each selection type.  This will also reduce
        the amount of code in the library and speed things up a bit more.

    Phase 3 will involve generating an MPI datatype for all types of selections,
        instead of only "regular" hyperslab and 'all' selections.  This will
        allow collective parallel I/O for all I/O operations which don't
        require type conversions.  It will also open up the door for allowing
        collective I/O on datasets which require type conversion.

    Phase 4 will involve changing the access pattern to deal with chunked
        datasets in a more optimal way (in serial).

    Phase 5 will deal with accessing chunked datasets more optimally for
        collective parallel I/O operations.

Platforms tested:
    FreeBSD 4.6 (sleipnir) w/ parallel & C++ and IRIX64 6.5 (modi4) w/parallel