GsfCombinedMaterialEffects, simplify the construction of the delta covariance

Christos Anastopoulosrequested to merge
ATLAS-EGamma/athena:GsfCombinedMaterialEffects_emplace_back_Covariance into master

Write the filling of the delta 5x5 covariance in a much simpler form

Edited by Christos Anastopoulos

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