Create a function that adds null nodes to networks with inferior number of vertices

[danielemule]

Given a sample of networks with different number of nodes, this function should add null nodes at the end of the networks that do not reach a certain maximum number of vertices. The maximum number of vertices can be given as an input by the user or it can be the maximum number of nodes reached by a network in the sample.

[danielemule]

I have created the add_null_nodes() function.
Moreover, I have created the nvd_num_vertices() function, that returns a summary related to the number of vertices in a sample (e.g., maximum number of vertices, mean, etc.).

You can find these functions in my graphmatch branch of nevada (link).