refactor!: Steppers use MagneticFieldProvider (2/2)

[paulgessinger]

Based on #666, this PR changes the way the steppers store and use the B fields. The B field template parameter goes away and they accept instances of std::shared_ptr<MagneticFieldProvider>. The use of std::shared_ptr should be fine, since the propagators and steppers are only rarely created, copied or destroyed. This change then also allows getting rid of BFieldVariant in the examples, since the generic MagneticFieldProvider can be used as an interchange type. Doing this reduces the compile time of two or our most compile-time resource hungry compilation units by over 20%. More improvements might be possible, but require more work.

There are some additional cleanups that become possible but I would like to keep this PR reasonably (to some extent) small, so those will be independent.

I did not observe decreases in propagation preformance.

	steppers take b field type			steppers take MagneticFieldProvider
test	master	any	sbo	sbo
prop	37.11 ms/event	38.09 ms/event	37.74 ms/event	35.51 ms/event
prop cov	74.44 ms/event	75.09 ms/event	74.29 ms/event	71.84 ms/event

This includes #666, #670, #673

BREAKING CHANGE: The stepper type structure changes. None of the steppers accept a B field template argument anymore. Constructors are changed to accept an std::shared_ptr<Acts::BFieldProvider> instead of an instance of the concrete B field type.

[paulgessinger]

@baschlag could give the vertexing based on this branch a shot and see if anything sticks out?

[codecov]

Codecov Report

Merging #675 (772b82a) into master (213d97d) will increase coverage by 0.02%.
The diff coverage is 79.16%.

@@            Coverage Diff             @@
##           master     #675      +/-   ##
==========================================
+ Coverage   48.94%   48.96%   +0.02%     
==========================================
  Files         325      325              
  Lines       16622    16639      +17     
  Branches     7760     7762       +2     
==========================================
+ Hits         8135     8147      +12     
- Misses       3039     3042       +3     
- Partials     5448     5450       +2     

Impacted Files	Coverage Δ
Core/include/Acts/Propagator/Propagator.ipp	37.33% <0.00%> (ø)
...clude/Acts/Vertexing/AdaptiveMultiVertexFinder.hpp	78.57% <0.00%> (ø)
...clude/Acts/Vertexing/AdaptiveMultiVertexFinder.ipp	42.19% <0.00%> (ø)
...re/include/Acts/Vertexing/ImpactPointEstimator.ipp	37.80% <0.00%> (ø)
...include/Acts/Vertexing/FullBilloirVertexFitter.hpp	66.66% <50.00%> (-13.34%)	⬇️
.../include/Acts/Vertexing/HelicalTrackLinearizer.ipp	29.78% <50.00%> (ø)
...clude/Acts/Vertexing/AdaptiveMultiVertexFitter.hpp	69.56% <66.66%> (+1.38%)	⬆️
...e/include/Acts/Vertexing/IterativeVertexFinder.hpp	82.35% <75.00%> (+9.01%)	⬆️
Core/include/Acts/Propagator/EigenStepper.ipp	51.88% <90.90%> (-0.04%)	⬇️
Core/include/Acts/Propagator/AtlasStepper.hpp	69.53% <100.00%> (-0.49%)	⬇️
... and 8 more

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[baschlag]

Hi, I finally managed to get this branch running in Athena and compared the Acts AMVF performance of this branch vs. the current default Acts AMVF in Athena (baseline). Physics performance (as expected) unchanged, CPU comparable to previous version (within measurement uncertainties I guess).

[paulgessinger]

Excellent news. This seems to be within precision, so we can go ahead with this one and #666

[paulgessinger]

Ok with #666 merged, this is ready for review.

[asalzburger]

I approve the changes here (there's one question regaring a TODO), I think we should create three follow-up PRs to clean out the unneccessary shorthand definitions, i.e. we I would do:

• PropataionTests/Example (@asalzburger)
• Fitter Tests/Examples (@XiaocongAi)
• Vertexing Tests/Examples (@baschlag)

We should basically make a rule that
using Stepper = Acts::EigenStepper<> should not be used, but e.g. `using KalmanFitter = Acts::KalmanFitter<Acts::EigenStepper<>, ..., ..>' is still fine.

What do you think?