Time slot also saves session. Booking method updated.
Session class changed to Abstract
Session no longer holds number of tutors
 Lab session holds number of tutors. Tutor functions assosciated with Lab Sessions
Deleted TimeSlot class(as it was kind of useless, repeated info in Session class) and updated Week and Day to reflect change
Fixed relationships between User, Tutor, Admin, Student and Lecturer.
Also removed/commented redundant code
Forgot to commit Student class(oops)
Added courseID to course class