Change python intro to use files

Changes to the Python Introduction chapter to use files when things get more complex.

This reshuffles the code editor chapter to between the Python installation and Python introduction chapters.

Reading through the tutorial, it felt like the interpreter was a good choice up until conditional loops and functions were introduced, so I didn't rewrite the whole thing. I did however add a small section on getting the code editor open and why we are switching to it at that point (see line 396 and onwards in python_introduction/README.md)

Closes #207 and #190

:gift: 
I thought about this, and I wondered if readers would be confused as to why we're sticking the function at the bottom. Now, I know that the tutorial is very hands-on and that the readers get to ask lots of questions, but I though adding in the ```if __name__ == '__main__':``` bit gave reason to it, instead of just extra code at the end of the file.

What do you think?