Constructor of o2::Header::DataDescription should pad with 0, not " " 

Right now we have that:

```
o2::Header::DataDescription::DataDescription("CLUSTERS") != gDataDescriptionClusters
```

because gDataDescriptionClusters is right-padded with spaces. I would suggest that either:

* The constructor pads with spaces rather than spaces.
* `gDataDescriptionClusters` and alikes are 0 padded (preferred).