Add templated mergers

Add templated SOACollectionMerger and VectorOfObjectsMerger, to make merging of SOACollection-based classes and std::vector<Object> collections easier.

Event/EventBase/include/Event/SOACollectionMerger.h

+/*****************************************************************************\
+* (c) Copyright 2000-2022 CERN for the benefit of the LHCb Collaboration      *
+*                                                                             *
+* This software is distributed under the terms of the GNU General Public      *
+* Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   *
+*                                                                             *
+* In applying this licence, CERN does not waive the privileges and immunities *
+* granted to it by virtue of its status as an Intergovernmental Organization  *
+* or submit itself to any jurisdiction.                                       *
+\*****************************************************************************/
+/** @class SOACollectionMerger.h SOACollectionMerger
+ *
+ *  Merge SOACollections into one. Template to work with all containers based on SOACollection.
+ *
+ *  @author Michel De Cian
+ *  @date   05/07/2022
+ */
+
+#include "LHCbAlgs/MergingTransformer.h"
+#include <string>
+namespace LHCb {
+
+  template <typename Type>
+  struct SOACollectionMerger final
+      : Algorithm::MergingTransformer<Type( const Gaudi::Functional::vector_of_const_<Type>& )> {
+    SOACollectionMerger( const std::string& name, ISvcLocator* pSvcLocator )
+        : Algorithm::MergingTransformer<Type( const Gaudi::Functional::vector_of_const_<Type>& )>{
+              name, pSvcLocator, {"InputLocations", {}}, {"OutputLocation", {}}} {}
+
+    Type operator()( const Gaudi::Functional::vector_of_const_<Type>& lists ) const override {
+
+      if ( lists.size() == 0 ) return {};
+
+      Type        out( lists[0].zipIdentifier(), lists[0].get_allocator() );
+      std::size_t overallSize = 0;
+      for ( const auto& list : lists ) overallSize += list.size();
+
+      out.reserve( overallSize );
+
+      std::size_t offset = 0;
+
+      for ( const auto& list : lists ) {
+
+        if ( list.empty() ) continue;
+        for ( const auto& obj : list.simd() ) {
+          out.template copy_back<SIMDWrapper::best::types>( list, offset, obj.loop_mask() );
+          offset += popcount( obj.loop_mask() );
+        }
+      }
+
+      return out;
+    }
+  };
+} // namespace LHCb