get_unprocessed_files does not work for multiple dataset per dataset_name

In the sample.json file, there can be multiple datasets per "dataset_name" field.

Since the get_unprocessed_files.py script retrieves the dataset information using the first file in the list of root files, datasets further away in the list will be ignored. We should make sure to retrieve the list of all the datasets per "dataset_name" field, and query their associated root files information using dasgoclient.