Test postfit

Created by: voisey

Closes https://github.com/NNPDF/nnpdf/issues/143

I've had a first go at creating a test for postfit. I would have liked to have spent more time on this before committing it but I wanted to push something before the weekend so I didn't forget about it. Right now I believe it has a flaw in that if you (i) run the test locally, (ii) already have the test fit downloaded and (iii) have rerun postfit on it having requested a different number of replicas to the original run (unlikely I know), then the assertion that the files should be the same in the new run and the run that already exists will fail. Btw, when adding an assertion I'm guessing it's good practice to add a message, e.g. assert x == 2, "x does not equal 2 and it should.". If so I can add those next week. Please feel free to edit/ add checks if you feel so inclined!