String. Possible values are "edgelist" and "gml".
If format = "edgelist", we just output the edgelist matrix contained in the PAFit_net object as it is.
If format = "gml", here is the specification of the gml file. There is a binary field directed indicating the type of the network (0: undirected, 1: directed). There are three atrributes for an edge: source, target, and time. There are three atrributes for a node: id, isolated (binary) and time. The atrribute time is NULL if the attribute isolated is 0 (since this is not an isolated node, we do not need to record its first apperance time). On the other hand, time is the node's appearance time if attribute isolated is 1.