Avoid error if ROOT install is broken
Add electron_spectrum_hess.txt to example data files
Add HESS electron spectrum systematic uncertainty band as txt file
Add HESS low-energy electron spectrum.
Add Fermi LAT electron spectrum as ascii file
Add Fermi electron spectrum data
Implement access to electron and TeV spectrum data
Implement cosmic_ray_flux function
Improve spectral model docstrings a bit
Convert docstrings in astropy sub-package to standard format.