start of work
Added make_colored_link
create_rooted_spanning_tree works.
create_rooted_spanning_tree divided in two.
fixed create_uncolored_tree
Added root parameter to create_rooted_spanning_tree.
Added post_order. Need direct_children function.
direct_children added.
post_order works.
Added list_descendants. Not tested yet.
Added number_of_descendants.
need to debug
number_of_descendants works.
Added find_search_set, need to debug.
find_search_set works
Added lowest_post_order. Need to debug.
Fixed. lowest_post_order works.
copied code to *.py file.
Corrected lowest_post_order to take into account all descendants instead of only the direct children.
Added highest_post_order.