Updated handling of decorations in PhotonPointingTool
A.k.a why decorations should be written by algorithms and not by tools
Updated the handling of decorations added by the PhotonPointingTool to
- make their name actually configurable
- not recompute them in case they are already available
FYI @gfrattar @jburzyns (Note that this is targeting main as I assume this is what you will use for the vertex studies, since the corresponding developments would introduce some FT0-violation.)