Fixes to Cluster Shape Variables monitored in EXO DQM + #1986 Code Review 

Based on pre11 + #1986:

This PR addresses the following issues:
- Fixes the shower shape issue found in the EXO DQM
   - https://twiki.cern.ch/twiki/bin/viewauth/CMS/Exo_700pre9GEDwrt700pre9_MC
   - This was caused by not using the rechit fractions to calculate these shower shapes.
- Some additional fraction clean-up (mean positions in 5x5s weren't using them)
   - Causes small changes in cluster covariances, but calculation is internally consistent now.
- Code Review : Uses faster calculation of pT and avoids lots of unneeded de-referencing.