Add type checking exception to node_attribute and link_attribute arguments of plot_network

[bernardoct]

If a Pandas DataFrame is passed as an argument to either by accident, an obscure exception from networkx saying it cannot convert something (a Pandas Series) to RGB is raised. It is difficult for the user to figure out what the problem is.

[kaklise]

Thanks for letting us know. WNTR should include more type checking in general. We'll work on that for the next release.