(consider to) implement flexible choice for nIter and approximate sincos/trigonometry via PropagationFlags

[slava77]

cmssw/RecoTracker/MkFitCore/interface/Config.h

Lines 8 to 12 in 36b9220

	struct PropagationFlags {
	bool use_param_b_field : 1;
	bool apply_material : 1;
	// Could add: bool use_trig_approx -- now Config::useTrigApprox = true
	// Could add: int n_prop_to_r_iters : 8 -- now Config::Niter = 5

So far we've been using the approximate sincos and did not see any problems. So, this may in the end stay, unless we suddenly decide to look at loopers or close to that.

There was also a longish-standing idea to improve the propToR to reduce the number of iterations. Depending on how that goes, perhaps nIter will actually be useful.

[slava77]

the issue is added in response to cms-sw#36546 (comment)