groupConnectivityNaive: add Zkn

groupConnectivityNaive.py

 from sage.graphs.graph import Graph, DiGraph
+from sage.rings.finite_rings.integer_mod_ring import Integers
+
+def Zkn(k, n):
+  Zk = Integers(k)
+  return Zk.cartesian_product( *([Zk] * (n - 1)) )
 
 def prettifyGraph(G_):
   """Convert graph into directed one and number its edges."""