Add configuration for merging EVNT_TR files

In ATLMCPROD-7665 it was noted that merging of EVNT_TR files in AthenaMP jobs fails. This is due to the fact that the EVNT_TR input/output file arguments are of type argPOOLFile and so try to use the default approach for merging reconstruction outputs. This merge request creates a new argEVNT_TRFile argument type where a specialized merging configuration is used. This should fix ATLASJT-407.