add network and bipartite network classes

[asizemore]

Resolves #229 Child issue of #234

This PR adds two new classes: network and bipartiteNetwork. Networks do not fit the statistical chart structure so they have their own base class network. As we add additional networks, we can further extend this class.

Important - this PR is meant to only add the appropriate classes. Will stack PRs for the methods but i want us to focus on getting the classes right first.

To dos for this PR:

• [x] decide what empty props should be, null or character or something
• [x] network class
• [x] validation functions

Intentionally left for later PRs

• real network methods
• add helpers for making random networks for testing
• getting data from whatever format into a network
• writing to json

[d-callan]

just going to comment here that we also talked about moving this to S4 right out the gate, and introducing an edgeList class so we leave clear room to grow into alternative data sources and structures. i think ill wait for those changes before reviewing this one.