GammaLib

The @Prefactor@ and @Index@ parameters for a diffuse map are biased. This can be seen in the following pull histograms:

| !map_prefactor.png! | !map_index.png! |

The model simulated here was the Cen A map with a Crab spectrum:
<pre>
<?xml version="1.0" standalone="no"?>
<source_library title="source library">
<source name="Crab" type="DiffuseSource">
<spectrum type="PowerLaw">
<parameter name="Prefactor" scale="1e-16" value="5.7" min="1e-07" max="1000.0" free="1"/>
<parameter name="Index" scale="-1" value="2.48" min="0.0" max="+5.0" free="1"/>
<parameter name="Scale" scale="1e6" value="0.3" min="0.01" max="1000.0" free="0"/>
</spectrum>
<spatialModel type="SpatialMap" file="cena_lobes_parkes.fits">
<parameter scale="1" name="Prefactor" min="0.001" max="1000.0" value="1" free="0"/>
</spatialModel>
</source>
<source name="Background" type="CTAIrfBackground" instrument="CTA">
<spectrum type="PowerLaw">
<parameter name="Prefactor" scale="1.0" value="1.0" min="1e-3" max="1e3" free="1"/>
<parameter name="Index" scale="1.0" value="0.0" min="-5.0" max="+5.0" free="1"/>
<parameter name="Scale" scale="1e6" value="1.0" min="0.01" max="1000.0" free="0"/>
</spectrum>
</source>
</source_library>
</pre>

This bug is possible related to #1248.