Add systematic errors in the error columns of the SED specifications about gamma-astro-data-formats

[micheledoro]

It's always good to add information, but then it must be clear. Combining systematics errors without knowing their statistical distribution is maybe a risk. Unless we assume (or impose) that they are normal too and somehow indipendent. For example, if you put atmospheric transmission as a systematics without considering that it bias also the energy you can get into trouble.

[cosimoNigro]

Hi @micheledoro, I thought the experiments could (optionally) provide a global estimate of the systematic uncertainties on flux points. How the different effects are combined into this estimate I think it is outside the scope of the specs.