Make the AthenaMTJ more complete before switching to it
This is preview MR that prepares AthenaMTJ for switching to it. There are two issues to resolve in separate MR. 1st is that TrackContainer needs the backends (Track and MTJ) to be served by the same type Holder. So we'll need the the Athena version of VectorTrackContainer (xAOD backend is in place already). Second is that the the MTJ with its 4 backends is quite tedious to register. I plan to experiment with a small helper struct that would simplify that.
Edited by Tomasz Bold