Network Layouts: expand backend support for network layouts #248

right now we can take a data table representing an edge list and a layout arg and make a network w the specified layout. some things we would also like to be able to do:

1. handle custom layouts, by extending the support for layouts to constructors other than the edge list based ones. these should take arbitrary coords.
2. update the layout after the network has already been created. that might mean introducing a new Network method like layout and a new Node method like updateProperty