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.