Major refactoring
Refactor add_car
Refactor exit_car
Add error message to exit_car
Cosmetic change to io_helper.h
Move clear_screen() and print_title() to io_helper
Call prompt_clear_screen() at program start
Cosmetic change
Move sort enum to io_helper.h
Rename members and member types of lot_t
Move sort enum to lot_t
Fix / reorder includes
Flush stream at end of get_input()
Put back newline at the end of date_t's >>