pixelPairStep addition

[mmasciov]

This is a draft PR for the addition of the pixelPairStep.
It has contributions by @leonardogiannini (who originally used highPtTripletStep windows for pixelPairStep).
I updated hit selection windows for pixelPairStep as for other iterations.

A comparison of the physics performance between the two sets of windows:
http://uaf-10.t2.ucsd.edu/~mmasciov/MkFit_mm/comparison_SIMVAL_iter6_hitselwindows/
-> Very slight difference in performance, with slightly more hits added in case of ad-hoc windows.

A comparison of the physics performance between the updated branch and CMSSW:
http://uaf-10.t2.ucsd.edu/~mmasciov/MkFit_mm/comparison_SIMVAL_iter6_hitselwindows_vsCMSSW/
-> Room for improvement for mkFit.

[makortel]

I suppose the addition of pixelPairStep needs to be propagated to the CMSSW setup.

[mmasciov]

@makortel: this is correct.
I can work on the CMSSW propagation if you wish, and let you review it, unless you prefer of course to propagate it in first person.

[makortel]

@mmasciov Please do, thanks!