public void AddGrade(string subjectName, double value, DateTime date)
        {
            Grade grade = new Grade(subjectName, value, date);

            _grades.Add(grade);
        }
 public void AddGrade(Grade grade)
 {
     AddGrade(grade.SubjectName, grade.Value, grade.Date);
 }
 public void DeleteGrade(Grade grade)
 {
     DeleteGrade(grade.SubjectName, grade.Value, grade.Date);
 }