Add the possibility to generate networks with random number of nodes

[danielemule]

It is useful to give the user the possibility to create a sample of networks with random number of nodes, in particular: N_1,N_2,...,N_{sample_size} iid Poisson(num_vertices).

[danielemule]

I have added this possibility to nvd(), via the rand_num_vertices argument.

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

[danielemule]

I have added the possibility to generate networks with random number of nodes following an Uniform distribution.

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

[astamm]

Please do open separate pull request for each of the issues instead of the links. Thanks!