Errata : The following numerical values should be quadrupled.
p.626, l. 1 : 6.25 and 15.7895
p.626, l. 3 & 4 : 12.7232
p.627, l.21 : 11.3258
p.627 Fig.3 : vertical scale and 12.7232 in the caption
p.628 Fig.4 : vertical scale and 11.3258 in the caption
The authors acknowledge Ms.Dora Ludvig and Dr. Peter Csoka for pointing out the errors.