[Bug] Impossible to create screen with numerous edges

User story :
As Tympan user, I want to be able to create screen with numerous edges, in order to be able to

Steps to reproduce behaviour :

• open Tympan
• openmodel.xml
• Launch acoustic computation
  • it fails with this error message : Error: Problème avec la surface Site 0->Infrastructure 0->Construction 14->Ecran mince 13-> :Face can not be triangulated (invalid plane?) => KO

Acceptance criteria :

• analysis docmented in this issue
• fix problem
• acoustic computation succeeds on the model attached to this issue