divergence attribute validation.

check_hy_graph() should run some optional validations... specifically, if a divergence attribute is included, it should check that the divergence attribute is valid and fully populated on the network.

Start by locating check_hy_graph() and inspect how divergence attributes are represented on the network. Determine the existing validation conventions and tests around this function; done means an included divergence attribute is checked for validity and completeness, while graphs without one continue through the optional validation.

Written by the indexing model from the issue text.