LCG_89/90a - finalize IO restructuring - change namespace ordering in FW

My apologies for this massive MR, but unfortunately the switch to the new DD4hep version was pretty much disruptive for testing (and left some of the other projects, i.e. I/O restructuring and FW namespace changes that had been started) untested.

I managed to merge those three branches, combine and test them agains my local updated DD4hep version, however to get this merged in, we need the CI to be updated to LCG 90a (@jhrdinka - can you confirm, please ?)

Can @rlangenb please update to the new LCG infrastructure and test ? I also updated the ROOT Cmake components, since they were incomplete.