Draft: bugfix in material collection

Sarka Todorovarequested to merge
todorova/athena:main-mat_blend into main

I'd suggest to consider using the blending utilities from GeoModelTool instead ( the interface can be adapted if need is ), see

https://gitlab.cern.ch/GeoModelDev/GeoModel/-/blob/main/GeoModelTools/GeoModelValidation/src/GeoMaterialHelper.cxx?ref_type=heads#L74

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