cross sections for nuclear primaries are calculated wrong for Sibyll

After Nikos' comments about iron showers in C8 I ran some tests and found that 100EeV iron primaries do not produce showers in C8... The reason is that the cross section is zero because the energy calculated in getCrossSection is wrong. At least this line according to the new interface does not calculate \sqrt{S_{NN}}

HEPEnergyType const sqrtSnn = (projectileP4 + targetP4).getNorm();

Not so surprising considering that the validation of the new interface is still pending...