Implement normalize attribute in GModelSpatialDiffuseMap.

The normalize attribute provides control over whether a map should be normalize or not. By default a map will be normalize to that the spectral model provides the correct fluxes, but for some specific analyses, normalization can now be switched off by adding normalize="0" in the XML file. The attribute has also be added as optional argument to the constructors. A unit test has been added that checks the functionnality.