Add an API to give checks and info on the taxonomy

[alexgarel]

Add an API to perform some checks on a taxonomy

The neo4J requests should be in entries.py, one method for each check /info.

checks:

• check that there is no cycle in the graph
  • if there is one, print the cycle

Info:

• total number of entries
• list of existing language with the % of translated entries
• list of root entries: (entries without parents)
  • with children
  • without children
• list of entries with the greatest number of direct children