enh: benchmark contours in hierarchy children

[paulmueller]

map_indices_child2parent might be better replaced with map_indices_child2root and keep a reference of the root contour object. Contours are nowadays ancillary features (computed from mask) and obtaining the contour ancillary feature via __getitem__ costs approximately 0.003 s, because the ancillary feature always has to check if it is available.

Thus, exporting contour data from hierarchy children is penalized with an overhead of 0.003 s per event.