Do not crash on missing edgelist

Motivation: there may be only one of the two edgelist files in the
current directory.

Analysis/report_graph.py

@@ -8,40 +8,44 @@ import csv
 import shapefile
 from os import path
 from networkx import nx_agraph
+import os
 
 def read_eddy_graph(edgelist, shp_tr_dir = None, read_interp = True):
     G = nx.DiGraph()
 
-    with open(edgelist) as f:
-        reader = csv.reader(f, delimiter = " ", skipinitialspace = True)
+    if os.access(edgelist, os.R_OK):
+        with open(edgelist) as f:
+            reader = csv.reader(f, delimiter = " ", skipinitialspace = True)
 
-        # Skip title lines:
-        next(reader) 
-        next(reader)
+            # Skip title lines:
+            next(reader) 
+            next(reader)
 
-        for row in reader:
-            k1, i1, k2, i2 = int(row[0]), int(row[1]), int(row[2]), int(row[3])
+            for row in reader:
+                k1, i1, k2, i2 = (int(row[0]), int(row[1]), int(row[2]),
+                                  int(row[3])
 
-            try:
-                weight = float(row[4])
-            except IndexError:
-                weight = None
-                
-            G.add_edge((k1, i1), (k2, i2), weight = weight)
+                try:
+                    weight = float(row[4])
+                except IndexError:
+                    weight = None
 
-    if read_interp:
-        # Read and set attributes of interpolated eddies:
+                G.add_edge((k1, i1), (k2, i2), weight = weight)
 
-        # Assuming that the directory containing the interpolated
-        # eddies is in the same location as edgelist:
-        extremum = path.join(path.dirname(edgelist), "SHP_triplet", "extremum")
-        
-        set_attribute(G, extremum)
+        if read_interp:
+            # Read and set attributes of interpolated eddies:
 
-    if shp_tr_dir is not None:
-        # Read and set attributes of Visible eddies:
-        extremum = path.join(shp_tr_dir, "extremum")
-        set_attribute(G, extremum)
+            # Assuming that the directory containing the interpolated
+            # eddies is in the same location as edgelist:
+            extremum = path.join(path.dirname(edgelist), "SHP_triplet",
+                                 "extremum")
+
+            set_attribute(G, extremum)
+
+        if shp_tr_dir is not None:
+            # Read and set attributes of Visible eddies:
+            extremum = path.join(shp_tr_dir, "extremum")
+            set_attribute(G, extremum)
 
     return G