Bulk grid submission

We should see if we can rewrite the grid submit script to use the bulk submission feature.

Right now we do some ugly stuff looping over datasets, but it looks like we should be able to just:

• make a json file listing all the datasets
• call prun --inOutDsJson test.json ...

The file is supposed to be formatted like

[
  {"inDS": "group.susy.abc/,group.susy.def/", "outDS": "user.hoge.XYZ"},
  {"inDS": "group.susy.opq/", "outDS": "user.hoge.VWX"}
]

I was reminded of this feature while looking at easyjet/easyjet#271 (closed) so if anyone gives this a try they should keep the developers of both frameworks in the loop.