Code review comment for lp:~fluidity-core/fluidity/fix-lagr-detectors

I added tests for detector checkpointing and the single lagrangian detector fix. The buffer corruption issue, however, cannot reliably be triggered and requires a lot of detectors; so I think it is impractical to test for this in small/medium tests.

I have also added Jean Mensa's fix for large detector output files, but again this seems like a corner case that is very hard to catch in buildbot.