Return None from `open_shpc` when no `ishape_last`

Return None from `open_shpc` when `ishape_last.txt` is not found,
instead of `[len(readers["extremum"]) - 1]`, as needed by
`plot_eddy_contours.py`. So that we can take another action in another
script.

Common/util_detect_eddies.py

@@ -36,7 +36,6 @@ def open_shpc(shpc_dir):
         ishape_last = np.loadtxt(fname, dtype = int, ndmin = 1)
     except OSError:
         print("Could not read", fname)
-        print("We will use all the shapes.")
-        ishape_last = [len(readers["extremum"]) - 1]
+        ishape_last = None
 
     return readers, d_init, ishape_last

Inst_eddies/Analysis/plot_eddy_contours.py

@@ -265,6 +265,11 @@ if __name__ == "__main__":
 
     readers, d_init, ishape_last \
         = util_detect_eddies.open_shpc(args.shpc_dir)
+
+    if ishape_last is None:
+        print("We will use all the shapes.")
+        ishape_last = [len(readers["extremum"]) - 1]
+        
     fig = plt.figure()
     src_crs = ccrs.PlateCarree()
     projection = ccrs.PlateCarree()