sage.graphs.generators.smallgraphs: start caching is_projective_planar()

The Petersen graph is known to be projective planar, so we can
pre-fill the cache with "True" when it is constructed.

Author: orlitzky

src/sage/graphs/generators/smallgraphs.py

@@ -4189,6 +4189,7 @@ def PetersenGraph():
     from sage.graphs.generators.families import GeneralizedPetersenGraph
     P = GeneralizedPetersenGraph(5, 2)
     P.name("Petersen graph")
+    P.is_projective_planar.set_cache(True)
     return P