Export functionality.

[PeterKraus]

Adds export functionality to tomato, leveraging yadg preset -pa and closing #19:

• by default, completed jobs are exported into a datagram and zip archive as results.{jobid}.[json/zip]
• the file prefix can be modified using tomato: output: prefix: str
• the timestamp can be completed using the status.json file
• the archive and datagram can be exported in a different folder than the cwd by using tomato: output: path: str

Note, this PR requires the dgbowl/yadg#57 PR to be merged, after which the setup.py needs to be modified to refer to a packaged version of yadg.