The coface roots of a given simplex sigma are the roots of subtrees
in the trie whose descendents (including the roots themselves) are cofaces of sigma.
This traversal is more useful when used in conjunction with other traversals, e.g. a preorder
or level_order traversal at the roots enumerates the cofaces of sigma.
coface_roots(st, sigma)the simplex tree to traverse.
simplex to start the traversal at.