sage.graphs.graph: make is_projective_planar() a @cached_method

The is_projective_planar() function can be very slow. The first
example in the documentation is the Petersen graph, on which the
method is taking over six minutes to arrive at the answer (True).
We mark it as a @cached_method, since that will allow us to pre-
fill the cache for graphs that are known to be projective planar,
saving the user a lot of time.

Author: orlitzky

src/sage/graphs/graph.py

@@ -428,6 +428,7 @@
 from sage.parallel.decorate import parallel
 from sage.misc.lazy_import import lazy_import, LazyImport
 from sage.features.mcqd import Mcqd
+from sage.misc.cachefunc import cached_method
 
 lazy_import('sage.graphs.mcqd', ['mcqd'],
             feature=Mcqd())
@@ -9541,6 +9542,7 @@ def bipartite_double(self, extended=False):
         G.name("%sBipartite Double of %s" % (prefix, self.name()))
         return G
 
+    @cached_method
     @doc_index("Graph properties")
     def is_projective_planar(self, return_map=False):
         r"""
@@ -9568,7 +9570,7 @@ def is_projective_planar(self, return_map=False):
         The Peterson graph is a known projective planar graph::
 
             sage: P = graphs.PetersenGraph()
-            sage: P.is_projective_planar()  # long time
+            sage: P.is_projective_planar()
             True
 
         `K_{4,4}` has a projective plane crossing number of 2. One of the