problem with EBL absorption

[jsitarek]

I tried to plot EBL absorption from agnpy, however got really strange results.
See the attached notebook.
ebl_absorption_check.ipynb.gz

I think that the underlying fits files with the EBL models are buggy.

In addition there are two problems:

1. if one tries to interpolate beyond the z range (or energy range) of the model it tries either way.
   I think that the extrapolation in energy should work such that it gives tau = 0 below the min energy and tau = infinity above the highest energy

2. the interpolation is done in the attenuation space, which can change very fast, I think it is much better choice to interpolate tau, and compute the absorption from it