Heavy-Ion reconstruction updates imported to 73X

The HI RECO updates that were previously implemented in 5_3_X are re-implemented in 7_3_X.

Replaces : 
https://github.com/cms-sw/cmssw/pull/5392/commits
and
https://github.com/cms-sw/cmssw/pull/4945 (72X)

```nullptr``` instead of 0