The writing of this paper started in 2004 when both G.M. and K.St. spent
a research period at the University of Erlangen-Nuremberg; the writing went on
over the years, with F.D. visiting the University of Parma, and G.M. the Univer-
sity of Erlangen-Nuremberg, a number of times: the support of such institutions
is hereby acknowledged. G.M has been partially supported by the minister of
higher education via the national project “Calcolo delle Variazioni” and by the
ERC grant 207573 “Vectorial problems”. Last but not least, special thanks go to
Verena ogelein for reading a preliminary version of the manuscript and for her
precious remarks.
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