Feature: Add order_by method to filter containers

[cortadocodes]

Contents

New Features

• Close order_by filtering for Dataset files #8 - order_by filtering for Dataset files
• WIP Queryset for dataset files - chainable filters  #6 - Queryset for dataset files, chainable filters

Minor fixes and improvements

• Raise a more helpful error message if trying to filter by a non-existent attribute of an item in a filter container

Quality Checklist

• New features are fully tested (No matter how much Coverage Karma you have)

Comments

• I don't think there's any need for a yield method as you can already yield from both a set and a list and they also already exist in memory so there's no benefit to returning a generator from them. Let me know if I've got the wrong end of the stick here