Release (parallel) timing code to upstream

[egpbos]

Make a feature branch for the (parallel) timing additions and merge it with upstream master:

• [ ] Make feature branch roofit_parallel_timing_clean
• [ ] Cherry pick the (parallel) timing commits that add the actual timing code from the roofit_parallel_timing branch:
  • [ ] RooTimer things
  • [ ] its use in RooRealMPFE and RooAbs(Opt)TestStatistic
  • [ ] search other uses and decide whether to add them
• [ ] Clean up the cherry picked code where necessary
• [ ] Check and/or create documentation for it.
• [ ] Create tests for it.
• [ ] Create pull request to upstream master.