Change type of property to `vector<double>`

This allows readability of the property in graphml format. As vector
of double is more specific than python object, I suspect this commit
also makes the code more efficient. See also commit c7f7fd46.

cost_function.py

@@ -110,8 +110,8 @@ t1 = time.perf_counter()
 timings.write(f"loading: {t1 - t0:.0f} s\n")
 t0 = t1
 
-g.vp['pos_first'] = g.new_vp('object')
-g.vp['pos_last'] = g.new_vp('object')
+g.vp['pos_first'] = g.new_vp('vector<double>')
+g.vp['pos_last'] = g.new_vp('vector<double>')
 g.vp['first_av_rad'] = g.new_vp('float')
 g.vp['first_av_ros'] = g.new_vp('float')
 g.vp['last_av_rad'] = g.new_vp('float')