[ATR-19881][ATR-20274] Improve interface of new associateToEventView helper function

This is a follow up to last week's !27749 (merged)

I realised (a bit too late for that MR) that the new interface still had a couple of areas in which it could be improved.

• It is better to consume a const SG::ReadHandle<CONTAINER>& than a const SG::ReadHandleKey<CONTAINER>& as with the latter, one also needs access to the EventContext to construct the actual handle.
• The function can be templated on the CONTAINER type rather than DataVector<OBJECT>, we don't need to directly assign the OBJECT. We can get away with a double-dereference instead (objectROI = accessor( **it ))

@jmasik - I have been working more on https://twiki.cern.ch/twiki/bin/view/Atlas/TrigDecisionToolMTFeatureAccess and I will document the interface as in this MR and then send out a wider mail.

Another small change added is a try-catch in the TrigEDMChecker.cxx as otherwise this gets tripped up by a photon chain which has an interface container but no aux. Will open a bug report for the underlying cause.