Get rid of all intermediate nodes that aren't needed.

def _prune(self):
        """ Get rid of all intermediate nodes that aren't needed. """
        done = False
        while not done:
            done = True
            for node in [node for node in self.nodes.values() if node.intermediate]:
                if not [dep for dep in self.nodes.values() if dep.previous == node.volume]:
                    # logger.debug("Removing unnecessary node %s", node)
                    del self.nodes[node.volume]
                    done = False