Add luminosity function for a parametrization of the observed blazar population.

[itaboada]

This is a paper for blazars: https://iopscience.iop.org/article/10.1088/2041-8205/800/2/L27

Warning: The Ajello 2012 and 2014 on this topic had a wrong sign in the function. It's described in Formulas 1-10

[mlincett]

Is this a duplicate of #71 ?

[itaboada]

Duplicate issue