Alpha release 0.6.0-alpha

This release includes some optimisations and under the hood improvements, readying for addition of a proper database.

Added

• Implemented temp directory removed on app exit (if it contains files), initially used to store avatars before writing to database/disk.
• NewClass subclass of Class using temp directory to hold files before writing to database.
• Add AllContributors badge to README.md, recognising project contributors, thankyou!

Changed

• Separate UI/logic/persistence concerns in create_classlist, use NewClass.
• create_classlist_data takes a NewClass object.
• Refactor new_chart/assemble_chart_data to take Class/NewClass object instead of a class name.
  • Supports passing in class directly from create_class without reloading class from database.
  • Move logic prompting user to choose a class from assemble_chart_data to new_chart.
• More tests converted to Pytest style tests, test coverage improved.
• Improved type hinting, extend passing/use of Paths rather than strings/os/path.