Freeze the new data format

[lucabaldini]

Ok, we should really move on and freeze the data format. There's a few aspect to this that we want to track in a consistent fashion:

• The file header, see issue #86
• Tracking the vref, which is probably calling for a buffer header, since the vref readout is in synch with the data buffers, see issue #137
• The error contribution which is supposed to come along with the new firmware.

And this is affecting everything downstream, primarily the reconstruction, so we don't want to change things every other day.