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)

you do not need the sqrt, use the squared values (faster);
all constants (like 3.0) are better to be named (const variables) or at least put a comment in the code suggesting what is the meaning;
use consistently std::abs() instead of fabs and abs;
could any of the denominators be 0?