Update text and comments of session_1

[corentinlger]

Description

Update notebook sessions for the course. At the moment I think we should :

• Write the core text of all notebooks
• Write all the responses to the questions (see how we remove them later)
• Write the text about installs, starting / stopping the processes, reloading the simulation if it crashes ... later

Done :

• Tried to simplify the API
  • only stop function for the controller (removed pause)
  • all functions printing something (for controller or entities) start with print_
  • start behaviors with the attach_behavior function by default to simplify the code
• Start updating session 1 content and comments
• Start updating session 2 content and comments
• Start updating session 3 content and comments