Save pixel width in Pixel Cluster EDM

Merged Carlo Varni requested to merge cvarni/athena:SavePixelWidth into master

Pixel Width is already computed during cluster creation but then it is not saved into the xAOD::PixelCluster EDM. This variable is later used during pixel space point formation, but we are obliged to re-compute that. That takes a considerable amount of time.

With this MR the variable is added to the EDM and later retrieved for the computation of the space point covariance. Huge gain in CPU for space point formation, no cost for pixel clusterization.

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