Be aware of the use of the gfapy python library

[vepain]

I raised several bugs and unexpected behaviour in the gfapy Python library:

From https://github.com/ggonnella/gfapy/issues?q=is%3Aissue%20author%3Avepain%20:

• Unexpected behaviour: the reverse of a link can be added to the graph ggonnella/gfapy#35
• Duplicate reverse loop links in dovetails properties ggonnella/gfapy#36

We have to make sure they do not affect our tool and its API.