Correcting the pims_split metadata and dag files to iterate over the splits

Added an iteration over {{#splits}} to the metadata file and removed an unnecessary check of the quicklook_url in the dag (it's okay to pass an empty string there).

The splits entry should look like:

"splits": [
    {
        "split_dir": {{split_dir}},
        "quicklook_url": {{quicklook_url}}
    },
]