'Returns a dictionary of dictionaries.
The primary dictionary is keyed by the chapter\'s usage_key.
The secondary dictionary contains the chapter\'s
subsection grades, display name, and url name.'

@lazy
def chapter_grades(self):

    course_structure = self.course_data.structure
    grades = OrderedDict()
    for chapter_key in course_structure.get_children(self.course_data.location):
        grades[chapter_key] = self._get_chapter_grade_info(course_structure[chapter_key], course_structure)
    return grades
