Bug fix: missing suffix

We have to precise the suffix because we test readability of file with
`os.access` below. The bug comes from commit 41c8c5cc.

Analysis/report_graph.py

@@ -38,7 +38,7 @@ def read_eddy_graph(edgelist, shp_tr_dir = None, read_interp = True):
             # Assuming that the directory containing the interpolated
             # eddies is in the same location as edgelist:
             extremum = path.join(path.dirname(edgelist), "SHP_triplet",
-                                 "extremum")
+                                 "extremum.shp")
 
             if os.access(extremum, os.R_OK):
                 set_attribute(G, extremum)