Adding vertex plotting option to plot_network

[kbonney]

Provide a summary of the proposed changes, describe tests and documentation, and review the acknowledgement below.

Summary

This PR adds the ability to plot vertices to the plot_network function.

Tests and documentation

Below are comparisons of the graphics generated in the current tests by plot_network with the same graphics generated with plot_vertices=True to verify that the updates don't break current behavior. Note, these don't actually plot any vertices because the involved networks don't have any.:

• test_plot_network1
• test_plot_network2
• test_plot_network3
• test_plot_network4
• test_plot_network5

A new test using io.inp is added to verify a network that actually have vertices:

TODO

• [ ] decide whether to make plot vertices default behavior or leave it as an optional argument.

Acknowledgement

By contributing to this software project, I acknowledge that I have reviewed the software quality assurance guidelines and that my contributions are submitted under the Revised BSD License.

[kbonney]

• [ ] Test computational load on large networks
• [ ] see if vertex graph generation could be moved to to_graph()