Henneaux, Marc
Lekeu, Victor
Leonard, Amaury
Article History
Received: 28 November 2017
Accepted: 8 January 2018
First Online: 23 January 2018
Change Date: 16 May 2018
Change Type: Erratum
Change Details: There are typos in equations (2.27), (A.15) and (C.22).