Added functionality to read energy boundaries from IRF Fits file as well from observation definition XML files
Avoid overflow in powerlaw flux computation.

The original powerlaw integration functions were prone to overflows for large gammas as the function computed the pow(energy, -gamma), where -gamma can be a large positive number.
Add powerlaw flux computation unit test.
Integrate code.
Merge branch '1039-allow-energy-thresholds-via-IRF-and-XML' into integration