This work has been carried out within the framework of the Contracts of Association between the European Atomic Energy Community and ``Instituto Superior T\'{e}cnico'' (IST) and IPP.CR. IST also received financial support from ``Funda\c{c}\~{a}o para a Ci\^{e}ncia e Tecnologia'' in the frame of the Contract of Associated Laboratory.
The views and opinions expressed herein do not necessarily reflect those of the European Commission. This work was also supported by Czech Science Foundation, grants 202/08/H057.