Rename Digitization.Pileup -> Digitization.PileUp and configure pile-up digi steering

Tadej Novakrequested to merge
tadej/athena:digi/flag into master

Configure pile-up digitization steering in the CA-based config.

Rename Digitization.Pileup to Digitization.PileUp to use more consistent naming (PileUp is used more commonly than Pileup).

Also add Digitization.JobNumber flag and autoconfigure Digitization.PU.BunchSpacing.

/cc @jchapman.

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