Release CPU affinity to upstream

[egpbos]

Make a feature branch for the CPU affinity addition and merge it with upstream master:

• Make feature branch roofit_cpu_affinity
• Cherry pick the CPU affinity commits that add the actual CPU affinity code in RooRealMPFE and the CPUAffinity option that is passed to the minimizer via fitTo().
• Check and/or create documentation for it.
• Create a test and maybe benchmark for it.
• Create pull request to upstream master.

[vincecr0ft]

I'm testing an implementation of the models found here in order to give both binned and unbinned benchmarks to support justification for CPU affinity.