Moved the method get instance to the collections module
Added attr_reader to the Subject class for the array of students
Added an instance method to the Student class to get the list of classes in which the student is registered
The @students property is now a hash, to store the grades of the students
Added a method to update the grade on a definedstudent
Bug fix
Added the option to update a student's grade on a defined subject to the menu
Merge pull request #14 from OscarSwanros/feature/add-final-score-to-student-on-subject

feature/add-final-score-to-student-on-subject