Major refactoring for statistics

[marfox]

statistics.py currently looks like a mix of functions and commands. Please tidy up the script by refactoring into small, well-documented functions. Use a main function as well.

[papalinis]

A major refactoring has been done to statistics.py, including:

• Usage of a main function
• Argparse to handle command line arguments
• Refactoring of most of the function involved in the script.

Please have a look at the script and tell me if now the things are clearer.
Hope you would help me improve my code style again,
Simone