Explicit StrebelGraph for each meromorphic stratum

[videlec]

Boissy and Chen-Gendron classified the connected components of the moduli space of Abelian and quadratic meromorphic differentials. These are in correspondence with the connected components of the Delaunay-Strebel automata. As in the holomorphic case in Zorich (implemented in surface_dynamics) we should provide for each stratum an explicit StrebelGraph.