Added Subject Table and Combobox.
To add teacher to the 3rd colum, select the appropriate cell then, select the teacher from the ComboBox. The teacher initials will automatically be inserted into the selected cell in the JTable.
Note that the ComboBox is initalized with the list of teacher intitials when the Subject Frame first loads.
To Do:
Will implement algorithm to allocate Subjects to Classroom.
Then will test the allocation algorithm.
If it works, I will add the Classroom Frame so that more classrooms may be added.
