Add write handle keys to the BCM_DigitizationTool (ATLASSIM-3979)

Tadej Novakrequested to merge
tadej/athena:overlay/bcm-workaround into master

Add write handle keys to the BCM_DigitizationTool to be able to output hard-scatter BCM information in overlay.

/cc @jchapman

Edited by Tadej Novak

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