'Get credit requirement of a given course.
Args:
course_key(CourseKey): The identifier for a course
namespace(str): Namespace of credit course requirements
name(str): Name of credit course requirement
Returns:
CreditRequirement object if exists'

@classmethod
def get_course_requirement(cls, course_key, namespace, name):

    try:
        return cls.objects.get(course__course_key=course_key, active=True, namespace=namespace, name=name)
    except cls.DoesNotExist:
        return None
