FatGraph enhancements

[videlec]

• more compatibility between fat graphs and 3-constellations
• support for folded edges (as in veerer). These correspond to fixed points for the edge permutation ep.
• add an edge flip operation (both forward and backward)
• more enumerators
  • face-bipartite and/or vertex-bipartite fat graphs
  • simple graphs (ie with/without loops and with/without multiple edges)
• uniform and Boltzman random fat graphs

[videlec]

One way to support folded edges (and canonical edge permutation) is to introduce two integers for the number of edges and number of folded edges. Canonical edge permutations are (0,1)(2,3)(4,5)(6)(7)(8) (folded edges at the end).