[A company's] "principal place of business" is . . . where a corporation's officers direct, control, and coordinate the corporation's activities. It is . . . called the corporation's "nerve center." And in practice it should normally be the place where the corporation maintains its headquarters—provided that the headquarters is the actual center of direction, control, and coordination, i.e., the "nerve center," and not simply an office where the corporation hold its board meetings (for example, attended by directors and officers who have traveled there for the occasion).