This MR bring the definition of the xAOD::BaseContainer. That is a container that has no actual content. It allows though to be freely decorates. It turned out that in several areas people abuse other containers for exactly that use-case, see discussion: https://groups.cern.ch/group/hn-atlas-offlineSWHelp/default.aspx?RootFolder=%2Fgroup%2Fhn%2Datlas%2DofflineSWHelp%2FLists%2FArchive%2FEMD%20class%20for%20an%20objet%20having%20only%20decorations&FolderCTID=0x012002002BD1887D3A9C0D489560C010AA1FE9F4&View={5121AF6B-9AFF-4225-8B52-9244D9184E7C}
Tagging people contributing to the discussion: @krumnack @jburr @strichte @mdyndal And core s/w team that will likely be very interested: @akraszna @ssnyder
It was tested in the "full" chain with writing AOD file and reading it back.