mohller/AstroPhoMes: AstroPhoMes v0.3

The code has been updated to reflect the state of the paper L. Morejon et al 2019 at the moment it was accepted for publication.

Changes from previous version:

- Corrected in EmpiricalModel class the pion inclusive cross section treatment for energies above 1 GeV (see paper)

- Included a test to evaluate the correctness of the implementation

- Modifications to the Howto_example.ipynb notebook to be consistent with the rest of the changes