Allow missing layer in class SHPC

Especially, we want to allow missing layer `max_speed_contour` when
running script `plot_eddy_contours.py`.

Common/util_eddies.py

@@ -65,8 +65,12 @@ class SHPC_class:
             my_shapefile = path.join(self.dir, f"Slice_{i_slice}", orientation,
                                      layer)
             print("Opening", my_shapefile, "...")
-            self._readers[i_slice][orientation][layer] \
-                = shapefile.Reader(my_shapefile)
+            try:
+                self._readers[i_slice][orientation][layer] \
+                    = shapefile.Reader(my_shapefile)
+            except shapefile.ShapefileException:
+                print("Not found")
+                self._readers[i_slice][orientation][layer] = None
 
         return self._readers[i_slice][orientation][layer]