Add models: [Manual]AxiomSelection There will be more subclasses of AxiomSelection each with different attributes. To avoid creating many NULL values in the database, we use multiple table inheritance with the gem 'acts_as_relation'. This gem mimicks actual class inheritance of ruby with the method `acts_as`. The migration changes the has_and_belongs_to_many association between Axioms and ProofAttemptConfigurations to be between Axioms and AxiomSelections.