We thank R. A. Pimenta, X. Martin, S. Nicolis, and G. Lemarthe for useful discussions. N.S. thanks the CNRS for his grant to join the Institute Denis Poisson of the University of Tours where a part of this work was done.
Поступила в редакцию: 29.08.2019 Исправленный вариант: 25.09.2019 Принята в печать: 27.09.2019