Marks a set of nodes returned by "get_ready" as processed.
This method unblocks any successor of each node in *nodes* for being returned
in the future by a call to "get_ready".
Raises ValueError if any node in *nodes* has already been marked as
processed by a previous call to this method, if a node was not added to the
graph by using "add" or if called without calling "prepare" previously or if
node has not yet been returned by "get_ready".