Add network analysis packages

[gvegayon]

This PR adds four R packages, 3 on CRAN and one on GitHub (to be submitted soon):

• rgexf and netplot for network visualization.
• epiworldR for fast Agent-Based models (network based, obviously)
• tabulergm (on GitHub) for creating fancy tables for ERGM models, including math and little graph representations of the sufficient statistics.

[jhollway]

Thanks George, I look forward to checking them out. Shall we wait until after CRAN holidays for tabulergm is on CRAN too (and thus can be on the CRAN TaskView)?

[gvegayon]

Surely! That's a better way to do it. I will submit tabulergm to CRAN; once that's done, I will update this PR and ping you.

Thanks!