Adding comments (tbc)

Just notes for me...
Enable user to exit program and write stuff to file

by typing "exit" the user can now exit the program (the while loop). This has implications for Studentin.write, which gets only executed once the program is properly exited (i.e. stuff is only written to my text file when I exit the program)
Add read and write methods to Studentin

tbc
Same has to happen for Courses and Studiengang