Add an API to perform checks and info on an entry

[alexgarel]

Add an API to perform some checks on an entry.
The requests should be in entries.py

checks:

• list of parents where one of the language of this entry is missing
• list of entries that have a synonym identical to this entry
• list of children that use a synonym instead of a main id to link to this parent (not sure we have the info)

info:

• shortest and longuest path to root
• number of descendants
• properties that are already defined on an ancestor and redefined in the children
• all inherited properties (of ancestor) with their values
• list of children than redefine one of the properties of this entry (and which one)