log-parabolic spectra

[GernotMaier]

Is there a possibility to add spectra of the following form?

dN/dE = F x (E/E_0) ^ ( -alpha - beta x log(E/E_0) )

Usually called log-parabolic form.

[cdeil]

@GernotMaier - Looks like we don't have one of those at the moment, but it's easy to add support for it in the gamma-cat schema file and scripts.

Can you please do the addition for this spectrum in a separate pull request, and then I'll update the schema of allowed spectral models and scripts in the coming days and merge it in.