deposit: FFT from arXiv

* Gets the FFT from arXiv if the arXiv id is given.
But I can only get the sip.metadata after the `prepare_sip` task... It's the only way to access the `metadata['arxiv_id']` and so retrieve the correspondent file. Can I simply run this on my task?

```
        fileinfo = {
            "type": "file",
            "filename": file_obj.name,
            "full_path": file_obj.get_syspath(),
        }
        obj.add_task_result(file_obj.name,
                            fileinfo,
                            "workflows/results/files.html")
```

Why can't the `add_files_to_task_results` task go after the `process_sip_metadata`?