Backward compatible changes to prepare for change in DataHandle option

Marco Clemencic requested to merge marcocle/prepare-gaudi-1144 into master

As agreed in gaudi/Gaudi#146 (closed), we will remove DataHandle.__add__ operators in Gaudi properties, implemented with gaudi/Gaudi!1144 (merged).

This MR fixes the few cases that were using DataHandle.__add__.

It's backward compatible, but mandatory in lhcb-gaudi-head, where gaudi/Gaudi!1144 (merged) is being validated.

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