Add EcalOverlap cluster collection type

Preema Rennee Paisrequested to merge
ppais_caloCovAlg into master

Added EcalOverlap cluster location (pre-requisite for Rec!1524 (merged), with CaloFutureShowerOverlap and CaloFutureClusterCovarianceAlg modified to use this location as output and input, respectively).

Edited by Marco Cattaneo

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