Fixed several bugs in the GUI

1) Fixed bug in heuristics that would cause a crash on new courses.
2) Added "enter" key functionality to student and assignment windows, so that pressing enter would either enter the current student name or home work assignment, or tab to the missing field.
3) Updated the assignment window to be consistent with the student window. Adding students through the table has been removed because it was causing a lot of bugs. The way I changed it is subtle, but it is quicker and has zero chance to bug out. Can still edit courses through the table.
4) Removed gradebooks/archive/records from the repo, no longer needed.
5) Program now creates file structure on start.

Also updated the add assignment menu to be consistent with the add student menu.