GammaLib

While finalizing the stacked cube analysis (#1291) I encountered a problem with fitting the radial disk model. For stacked cube analysis, the fit converges badly. But even event for unbinned analysis, the fit takes many iterations. Here for example a log for unbinned fitting:
<pre>
>Iteration 0: -logL=34182.372, Lambda=1.0e-03
>Iteration 1: -logL=34180.081, Lambda=1.0e-03, delta=2.290, max(|grad|)=19.555723 [DEC:1]
>Iteration 2: -logL=34180.070, Lambda=1.0e-04, delta=0.011, max(|grad|)=-6.847438 [Radius:2]
>Iteration 3: -logL=34180.070, Lambda=1.0e-05, delta=0.001, max(|grad|)=2.516642 [Radius:2]
>Iteration 4: -logL=34180.070, Lambda=1.0e-06, delta=0.000, max(|grad|)=-3.717756 [Radius:2]
Iteration 5: -logL=34180.070, Lambda=1.0e-07, delta=-0.000, max(|grad|)=3.502109 [Radius:2] (stalled)
Iteration 6: -logL=34180.070, Lambda=1.0e-06, delta=-0.000, max(|grad|)=3.502427 [Radius:2] (stalled)
Iteration 7: -logL=34180.070, Lambda=1.0e-05, delta=-0.000, max(|grad|)=3.501858 [Radius:2] (stalled)
Iteration 8: -logL=34180.070, Lambda=1.0e-04, delta=-0.000, max(|grad|)=3.501880 [Radius:2] (stalled)
Iteration 9: -logL=34180.070, Lambda=1.0e-03, delta=-0.000, max(|grad|)=3.499197 [Radius:2] (stalled)
Iteration 10: -logL=34180.070, Lambda=1.0e-02, delta=-0.000, max(|grad|)=3.455784 [Radius:2] (stalled)
Iteration 11: -logL=34180.070, Lambda=1.0e-01, delta=-0.000, max(|grad|)=2.826601 [Radius:2] (stalled)
Iteration 12: -logL=34180.070, Lambda=1.0e+00, delta=-0.000, max(|grad|)=2.395730 [Radius:2] (stalled)
>Iteration 13: -logL=34180.070, Lambda=1.0e+01, delta=0.000, max(|grad|)=-3.178778 [DEC:1]
Iteration 14: -logL=34180.070, Lambda=1.0e+00, delta=-0.000, max(|grad|)=2.706520 [DEC:1] (stalled)
Iteration 15: -logL=34180.070, Lambda=1.0e+01, delta=-0.000, max(|grad|)=-1.960376 [RA:0] (stalled)
>Iteration 16: -logL=34180.070, Lambda=1.0e+02, delta=0.000, max(|grad|)=-1.937920 [RA:0]
</pre>
The fit stalls often (which is also the case for binned and stacked cube analysis) and takes many iterations to terminate.