The first author has been supported by grants of RFBR 14-01-00160 and 15-01-02158, and by the Government of the Russian Federation within the framework of the implementation of the 5-100 Programme Roadmap of the National Research University Higher School of Economics, AG Laboratory. The second author has been supported by the Hermann–Minkowski–Minerva Center for Geometry at the Tel Aviv University and by the German–Israeli Foundation Grant No. 1174-197.6/2011. Main ideas behind this work have appeared during the visit of the second author to the Higher School of Economics (Moscow). We are grateful to these institutions for support and excellent working conditions.
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