Adding ONNX compatability to FastCaloSim

Spiritual successor to !66216 (closed) Which is itself a variant on !63562 (merged). Look to !63562 (merged) for the best documentation.

This is a slightly abbreviated version of the above MR's because there is a problem somewhere in the changes to TFCSPredicExtrapWeights.cxx. This problem caused the CI to fail. The problem is quite specific, it mostly doesn't show up. TO reproduce it outside the CI, you can run the 23.0 branch (which already has the changes, because it has less CI on it);

asetup Athena,23.0,latest && \
ATHENA_CORE_NUMBER=0 Sim_tf.py  --AMIConfig a913 --inputEVNTFile /cvmfs/atlas-nightlies.cern.ch/repo/data/data-art/CampaignInputs/mc23/EVNT/mc23_13p6TeV.601229.PhPy8EG_A14_ttbar_hdamp258p75_SingleLep.evgen.EVNT.e8514/EVNT.32288062._002040.pool.root.1 --outputHITSFile myHITS.pool.root --imf False  --maxEvents 20

So I took out the changes to TFCSPredictExtrapWeights.cxx, that now only runs on lwtnn. This MR only changes the net in TFCEGANEtaSlice.cxx so that it may run on lwtnn or ONNX. This bit seems to work just fine.

As I'm leaving, I won't be able to interact with gitlab. I'm still happy to help, if there is anything you'd like to ask; henrydayhall@protonmail.com