Fit correctly J0 from ICRC17 energy spectrum

DSPECTRUM_ANALYTIC_15 = np.array([3.3e-19, 4.82e18, 42.09e18, 3.29, 2.6, 3.14]) DSPECTRUM_ANALYTIC_17 = np.array([1, 5.08e18, 39e18, 3.293, 2.53, 2.5])

publication does not state J0 -> if needed could be fitted with data from:

https://www.auger.unam.mx/AugerWiki/SpectrumICRC2017

does not matter for rand_energy_from_auger, just scaling factor

Should be of the order of 3.3e-19