Implement stacked cube analysis of extended sources.

This is still pretty slow code, will now dive into this to better understand what the bottle necks are.
Remove GCTASourceCubeRadial
Refactor PSF integration for stacked cube analysis.
Update Doxygen documentatuion of romb() method.
Implement adaptive Simpson's rule integrator.

A new integrator has been implemented together with a unit test.
Update Doxygen documentation.
Implement Gauss-Kronrod quadrature in GIntegral class.
Set also relative error if information is available.
Romberg integrator now also sets relative and absolute error.
Add shell and disk model test data.
Implement and optimize stacked cube response computation for radial models.
Update to correctly set deadc and livetime.
Add elliptical disk model test data.
Correct deadtime corrections and recompute exposure and PSF cubes after taking into account 5% of deadtime.
Update unit test references after updating CTA test data including 5% deadtime.
Add IRF computation benchmark script
Test binned maximum likelihood fit.