Add dump adios to PDF build. Update example config with better documentation

The number of atoms per snapshot CAN change with the adios style.

When using the ADIOS tool 'bpls' to list the content of a .bp file,

bpls will print "__" for the size of the output table indicating that

bpls will print {__} for the size of the output table indicating that

its size is changing every step. 

The {atom/adios} and {custom/adios} dump styles are part of the USER-adios package. 

dimension.html

displace_atoms.html

dump.html

dump_adios.html

dump_h5md.html

dump_image.html

dump_modify.html

        <engine type="BPFile">

            <parameter key="substreams" value="10"/>

        </engine>

        <engine type="HDF5">

        The 'substreams' parameter in BPFile controls how many 

        files on disk are created. This number should be proportional 

        to the number of servers in the parallel file system, 

        NOT to the number of processes. 

        substreams=1 is generally a very inefficient setting for large parallel runs.

    -->

    <!--====================================================

    <io name="atom">

        <engine type="BPFile">

            <parameter key="substreams" value="1"/>

        </engine>

    </io>

    <io name="custom">

        <engine type="BPFile">

            <parameter key="substreams" value="1"/>

        </engine>

    </io>