Updated notebook to use the latest version of ALIAS

[mszczot]

• Added str method to Argument class
• Added the following to ArgumentationFramework class:
  • contains - to assert if the argument is present in the framework
  • iter - to allow to iterate easily through framework arguments
  • getitem - to allow indexing ArgumentationFramework with argument names
  • get_attackers - to get attackers for specified argument
• Updated Notebook to reflect the current implementation of ALIAS (all the functions above have been referenced in the Notebook. Classes have been extended to minimise the changes required for Notebook)