Defined method to add a student to a subject
Collections now returns each instance of the class in format array
Added a method that returs a Subject object based on an Id
Added an array of the students that attend the subject. Instance method for adding a student to a specific subject
Added the option to add a student to a subject to the menu.
Merge pull request #12 from OscarSwanros/feature/add-student-to-subject

feature/add-student-to-subject