hdf5doc

Purpose:
    SZIP and more general filter updates

Description:
    Added SZIP to the introductory paragraphs and as appropriate in
        the Fortran subroutine descriptions.
    Revised the introductory discussion, which had previously focussed
        on one compression filter, to allow for multiple filters of
        different types.
    Added list of filters currently distributed with HDF5 to intro.
    Also some copy edits and minor formatting.

Platforms tested:
    Mozilla, Safari