Use fewer variables

Rename `first_SHPC` and `last_SHPC` to `i_SHPC`. Rename `first_loc`
and `last_loc` to ishape.

cost_function.py

@@ -147,20 +147,18 @@ for n in g.vertices():
     # position of the extremum and store it in a vertex property:
     date_index, eddy_index = report_graph.node_to_date_eddy(segment[0],
                                                             e_overestim)
-    first_SHPC = get_SHPC(array_d_init, date_index)
-    first_loc = util_eddies.comp_ishape(handlers[first_SHPC], date_index,
-                                        eddy_index)
-    g.vp.pos_first[n] = handlers[first_SHPC]["readers"]["extremum"]\
-        .shape(first_loc).points[0] # [in degrees]
+    i_SHPC = get_SHPC(array_d_init, date_index)
+    ishape = util_eddies.comp_ishape(handlers[i_SHPC], date_index, eddy_index)
+    g.vp.pos_first[n] = handlers[i_SHPC]["readers"]["extremum"].shape(ishape)\
+                                                    .points[0] # [in degrees]
 
     # Same for last instantaneous eddy in the segment:
     date_index, eddy_index = report_graph.node_to_date_eddy(segment[-1],
                                                             e_overestim)
-    last_SHPC = get_SHPC(array_d_init, date_index)
-    last_loc = util_eddies.comp_ishape(handlers[last_SHPC], date_index,
-                                       eddy_index)
-    g.vp.pos_last[n] = handlers[last_SHPC]["readers"]["extremum"]\
-        .shape(last_loc).points[0] # [in degrees]
+    i_SHPC = get_SHPC(array_d_init, date_index)
+    ishape = util_eddies.comp_ishape(handlers[i_SHPC], date_index, eddy_index)
+    g.vp.pos_last[n] = handlers[i_SHPC]["readers"]["extremum"].shape(ishape)\
+                                                    .points[0] # [in degrees]
 
     if (num_of_days > num_of_days_to_avg):
         # The segment is longer than the number of days over which to average