returns one of the orders of the nodes such as the parents of any
node are less ranked than it when
is.null(ord). If not check
that the proposed order is either a right permutation
is.numeric(ord)) or a vector of node names providing a
topological order (
!is.null(ord) the order must be an order, if not an error
a permutation vector of the nodes of the /nbn/ or a named list with the nodes not having their parents before them.