* Issue #253: Tried to improve grade school structure. Removed .db() function, added numberOfStudents functions and renamed the sort function to studentsByGradeAlphabetical. Added test to check that grade() gets the students in the order they were inserted. Removed getsStudentsInAGrade() test as what it was testing was already being covered by the tests before it.

* Added tests for protection agains mutation of the list object

* Issue #253: Make the sort test more generic. This to allow a broader range of implementations. Currently users are hindered into return the type HashMap<Integer, List<String>. This test update will check the order of any Map that maps to any Collection value type. This opens the doors for more possible types of solutions.