Made the "xAOD ntuple writer" work with containers read from the input file.
This is to fix the issue discovered by @turra, and described in ATLASG-2451, that variables on containers that are read directly from an input file, would not be usable with the algorithm. As described in ATLASG-2451, this shortfall was introduced in !55866 (merged), while making the algorithm work correctly with ConstDataVector types.
Also extended one of the existing unit tests to be sensitive to this specific type of issue in the future.
Pinging @fwinkl.