Changing os.system -> subprocess

[ARiisgaard]

@crstn

I stumpled upon a stackoverflow answer indicating that subprocess was better to use than os.system. (https://stackoverflow.com/questions/44730935/advantages-of-subprocess-over-os-system)

Since os.system have a bit uncoopative so far I was considering rewriting the code based on subprocesses. However in the spirit of failing fast I just wanted to check that the code wasn't build on os.system because it needed something that subprocess' could not deliver