(minor) EBL absorption for E>1.66 TeV

[mireianievas]

This a minor improvement/suggestion. I'm working with some extrapolation of LAT spectra up to very high energies (for now up to 2 TeV), but if absorbed models are included (e.g. Franceschini 2008) in the analysis then the fermitools complain that the absorption table does not reach those really high energies (it indeed finishes at 1.66 TeV). I would suggest extending to higher values. For now I just included by hand one extra line to cover that 2 TeV in fermitools/data/eblAtten/opdep_Franceschini_2008.dat, but maybe it is worth to include a bit more lines in those files or allow the software to extrapolate?.

2.00E+03 1.33E-01 4.06E-01 1.42E+00 4.75E+00 8.65E+00 2.00E+01 3.13E+01 4.12E+02 5.52E+01

(the first line is the energy in GeV, the others are tau values for z=0.01, 0.03, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, 3.0)

The last value is kind of extrapolated by hand since the model does not reach z=3.0. I guess that was also done for lower energies by someone ???