Consider using argparse in the various scripts

[devos50]

The built-in argparse library is able to replace a lot of code in various Python scripts, such as generate_graph.py. It would also print a prettier message compared to the following:

mdevos@IC-ITs-MacBook-Pro-2 decentralizepy % python generate_graph.py       
Traceback (most recent call last):
  File "/Users/mdevos/Documents/decentralizepy/generate_graph.py", line 40, in <module>
    assert len(sys.argv) >= 2, __doc__
AssertionError: Usage: python3 generate_graph.py -g <graph_type> -n <num_nodes> -s <seed> -d <degree> -k <k_over_2> -b <beta> -f <file_name> -a -h