CSO - get new category tree
Following #631
CSO website changed a lot, and the category tree can't be parsed using the same technique as before. Until now using the category tree was the only way to get the full datasets list. Temporary solution used in #631 was to fix category tree using the last downloaded dataset (before the website changed).
Consequences:
- we probably do not provide all datasets
- if new datasets appears, we won't include them in DBnomics
- the category tree present in DBnomics is not up to date
We have to find a way to get the new category tree (I've done some promising tests).