Add data check and auto truncate for ProjectJudge

import cn.edu.sustech.cs307.dto.grade.Grade;

import cn.edu.sustech.cs307.dto.prerequisite.Prerequisite;

import cn.edu.sustech.cs307.factory.ServiceFactory;

import cn.edu.sustech.cs307.service.CourseService;

import cn.edu.sustech.cs307.service.DepartmentService;

import cn.edu.sustech.cs307.service.StudentService;

import cn.edu.sustech.cs307.service.UserService;

import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.File;

    private static final File enrollCourse2Dir = new File("./data/enrollCourse2/");

    private final ServiceFactory serviceFactory = Config.getServiceFactory();

    private final CourseService courseService = serviceFactory.createService(CourseService.class);

    private final DepartmentService departmentService = serviceFactory.createService(DepartmentService.class);

    private final StudentService studentService = serviceFactory.createService(StudentService.class);

    private final UserService userService = serviceFactory.createService(UserService.class);

    private final DataImporter importer = new DataImporter();

    public int testSearchCourses(File searchCourseDir) {

    }

    public void benchmark() {

        if (!courseService.getAllCourses().isEmpty()

                || !departmentService.getAllDepartments().isEmpty()

                || !userService.getAllUsers().isEmpty()) {

            System.out.println("Database is not empty! Please truncate (cascade) all your tables.");

        }

        // 1. Import everything other than studentCourses.json

        List<Department> departments = readValueFromFile("departments.json", List.class);

        List<Major> majors = readValueFromFile("majors.json", List.class);

        // TODO: Multi-threaded benchmark

        try {

            System.out.println("Truncate database");

            courseService.getAllCourses().parallelStream().forEach(it -> courseService.removeCourse(it.id));

            departmentService.getAllDepartments().parallelStream()

                    .forEach(it -> departmentService.removeDepartment(it.id));

            userService.getAllUsers().parallelStream().forEach(it -> userService.removeUser(it.id));

        } catch (Throwable t) {

            System.out.println("Failed to truncate database.");

            t.printStackTrace();

        }

    }

    public static void main(String[] args) {