Sexual networks

[robynstuart]

Add more examples, including examples of importing a network from elsewhere

[luojun-yang]

Connectors between network instances and between networks and disease modules.

[daniel-klein]

Networks should be able to make and manage multiple dynamic layers. This would enable instantiation of several independent layers or more complex algorithms with detailed connections between layers. In this approach, the connections between layers would simply be hard-coded as part of the more complex network algorithms.

[daniel-klein]

With #142 now merged, we have a base Network (algorithm) class and an example that imports a network from elsewhere (static networkx graphs). There is more to do in sexual networks, but I believe we have completed the task as described here..