Exercise.java UPDATE:
	- new variables
	- Cloneable implementation
DatumGC.java UPDATE:
	- Cloneable implementation
	- fixes
SimpleExercise.java UPDATE:
	- fixes
QuizExercise.java UPDATE:
	- Cloneable implementation
Student.java UPDATE:
	- Cloneable implementation
Teacher.java(enum) UPDATE:
	-fixes
+ update of  corresponding test classes