refactor!: B field access returns Result

[paulgessinger]

Previously, we didn't have a mechanism to communicate that a field query failed. This is problematic, because it's not clear what to do when for instance our interpolated field goes out of bounds (see #784). This PR makes MagneticFieldProvider require the getField and getFieldGradient methods to return Result<Vector3> instead of the bare vector. This changes the public interface slightly, and all clients need to be adjusted.

Additionally, the InterpolatedBFieldMap now checks each time a field cell is created or if the field is queried directly, whether the query position is out of bounds of the map, and will return a new MagneticFieldError::OutOfBounds if that happens.

BREAKING CHANGE:

• The signature of field query methods in MagneticFieldProvider changes from

  virtual Vector3 getField(const Vector3& position, Cache& cache) const = 0;
  virtual Vector3 getFieldGradient(const Vector3& position, ActsMatrix<3, 3>& derivative, Cache& cache) const = 0;

  to

  virtual Result<Vector3> getField(const Vector3& position, Cache& cache) const = 0;
  virtual Result<Vector3> getFieldGradient(const Vector3& position, ActsMatrix<3, 3>& derivative, Cache& cache) const = 0;

• InterpolatedBFieldMap::getMin and InterpolatedBFieldMap::getMax now return the extent of the valid interpolation domain, rather than the raw grid extent.

[codecov]

Codecov Report

Merging #825 (6ed2a7f) into main (bdd59e0) will decrease coverage by 0.13%.
The diff coverage is 26.31%.

@@            Coverage Diff             @@
##             main     #825      +/-   ##
==========================================
- Coverage   48.68%   48.54%   -0.14%     
==========================================
  Files         327      328       +1     
  Lines       16893    16958      +65     
  Branches     7945     7999      +54     
==========================================
+ Hits         8224     8233       +9     
- Misses       3066     3082      +16     
- Partials     5603     5643      +40     

Impacted Files	Coverage Δ
Core/include/Acts/MagneticField/SolenoidBField.hpp	100.00% <ø> (ø)
Core/include/Acts/Propagator/Propagator.ipp	37.17% <0.00%> (ø)
.../include/Acts/Propagator/detail/LoopProtection.hpp	42.10% <0.00%> (-17.90%)	⬇️
.../include/Acts/Vertexing/HelicalTrackLinearizer.ipp	27.83% <0.00%> (-1.96%)	⬇️
...re/include/Acts/Vertexing/ImpactPointEstimator.ipp	36.25% <0.00%> (-0.65%)	⬇️
Core/src/MagneticField/MagneticFieldError.cpp	20.00% <20.00%> (ø)
Core/include/Acts/Propagator/AtlasStepper.hpp	68.55% <23.52%> (-0.82%)	⬇️
Core/include/Acts/Propagator/EigenStepper.ipp	49.61% <24.00%> (-3.49%)	⬇️
Core/include/Acts/MagneticField/ConstantBField.hpp	70.00% <25.00%> (-10.00%)	⬇️
Core/include/Acts/MagneticField/NullBField.hpp	50.00% <25.00%> (-12.50%)	⬇️
... and 7 more

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[paulgessinger]

Turns out the interpolated b field test was actually wrong.
It was setting up the map like

detail::EquidistantAxis r(0.0, 4.0, 4u);
detail::EquidistantAxis z(-5, 5, 5u);

and then asserted that the map went from r=0-4 and z=-5,5, when in fact it went from r=0-3 and z=-5,3 because the upper edge of the outer bins doesn't have a value anymore, so the interpolation domain ends one bin lower. This PR now fixes this test.

Also, InterpolatedBFieldMap::isInside, getMin, getMax and InterpolatedBFieldMapper::isInside reported this wrong interpolation domain based on the raw extent of the underlying grid. Those should all be fixed now.

[paulgessinger]

Ok, I think now it should be ready, sorry for the flip flop @HadrienG2

[HadrienG2]

SGTM