Do not crash on missing SHPC directory

Do not crash on missing SHPC directory for interpolated
eddies. Motivation: when we call `read_eddy_graph` from
`plot_traj.py`, we just try to read data on interpolated eddies.

Analysis/report_graph.py

@@ -33,14 +33,18 @@ def read_eddy_graph(edgelist, shp_tr_dir = None, read_interp = True):
                 G.add_edge((k1, i1), (k2, i2), weight = weight)
 
         if read_interp:
-            # Read and set attributes of interpolated eddies:
+            # Try to read and set attributes of interpolated eddies:
 
             # 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 os.access(extremum, os.R_OK):
+                set_attribute(G, extremum)
+            else:
+                print("Trying to read data on interpolated eddies but ",
+                      f"{extremum} is not readable")
 
         if shp_tr_dir is not None:
             # Read and set attributes of Visible eddies: