**Acknowledgements**

T.S. Kavetskyy acknowledges the SAIA for scholarships in the Institute of Physics of SAS within the National Scholarship Program of the Slovak Republic. This work was also supported in part by the SFFR of Ukraine (Nos. F40.2/019 and F52.2/003) and MES of Ukraine (Nos. 0114U002616 and 0114U002617). A.L. Stepanov thanks the RFBR (No. 15-48-02525) and RSF (No. 14-13-00758) in Russia for financial support.
