The first author was supported by the Russian Federation Government Grant No 14.W03.
31.0031. T. U. would like to acknowledge support by the DFG Ul-403/2-1. T. U. also thanks
Winfried Sickel and Thomas Kuhn for several discussions on the topic. Last but not least T. U. ¨
would like to thank Kevin Matthes who implemented parts of the manuscript in TeX.