Rename variable `num_of_days` to `len_seg`

cost_function.py

@@ -131,7 +131,7 @@ print("Iterating on vertices...")
 for n in g.vertices():
     if n.in_degree() != 0 or n.out_degree() != 0:
         segment = g.vp.inst_eddies[n]
-        num_of_days = len(segment)
+        len_seg = len(segment)
 
         # Calculate the date index, the eddy index and the SHPC index of
         # the first instantaneous eddy in the segment, then grab the
@@ -153,7 +153,7 @@ for n in g.vertices():
         g.vp.pos_last[n] = handlers[i_SHPC]["readers"]["extremum"]\
             .shape(ishape).points[0] # [in degrees]
 
-        if (num_of_days > n_days_avg):
+        if len_seg > n_days_avg:
             # The segment is longer than the number of days over which
             # to average