wait for data to arrive at all in-ports before starting process
_IIP_: removed, Merge: dont wait for all in-ports to have data, Subnet: Pass sub-network the parent process open connections & close in-port and out-port on the exported sub-network
bring back connection and exported-port iterators to extract graph edges, support exported interface for composite-components, deliver IIPs at graph-edge creataion-time, return network representation as dict, handle closing connections on the exported interface for composite-components
no longer creating special _IIP_ process, insert Merge components for multi-connections involving IIP and exported interface
simplified fix for Pipe() file descriptor leak
added pygraphviz dependency