feat: switch to DormandPrince745 stepper by default

[wdconinc]

Briefly, what does this PR introduce?

This PR changes the default stepper from:

  stepper = "ClassicalRK4"
  eps_min = 5e-05
  eps_max = 0.001
  delta_chord = 0.25 * g4units.mm
  delta_one_step = 0.01 * g4units.mm
  delta_intersection = 0.001 * g4units.mm
  min_chord_step = 0.01 * g4units.mm

to

  stepper = "DormandPrince745"  ## different
  eps_min = 5e-05
  eps_max = 1e-04  ## 10x smaller
  delta_chord = 0.025 * g4units.mm  ## 10x smaller
  delta_one_step = 0.01 * g4units.mm
  delta_intersection = 0.01 * g4units.mm  ## 10x larger
  min_chord_step = 0.01 * g4units.mm

The differences are for the following reasons:

• DormandPrince745 has 6 evaluations per step instead of ClassicalRK4 which has 11 per step
• eps_max is reduced because it sets the relative scale of maximum accumulated step errors, and for a 2 m radius 1 GeV particle in a 1.7 T field this results in 2 mm, much larger than the pixel size,
• delta_chord is reduced because it is the sagitta between the step chord and the true curved track, and at 0.25 mm this is much larger than a pixel,
• delta_intersection is increased because a precision of 1 um is much smaller than the size of pixels we care about.

Note: to explore these on the command line, pass them as bare numbers in mm without explicit units, e.g. --field.delta_chord=0.025.

This PR, in preliminary testing (with tracking_only) increases throughput by about 5%.

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue: different stepper)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

No.

Does this PR change default behavior?

No.

[Copilot]

Pull request overview

This PR updates the npsim front-end defaults for Geant4 magnetic field integration to use the DormandPrince745 stepper and retunes several field accuracy/geometry tolerances to better match pixel-scale detector resolutions.

Changes:

• Switch default magnetic field stepper to DormandPrince745.
• Tighten eps_max and delta_chord, and loosen delta_intersection to new target tolerances.
• Introduce g4units usage for mm-based configuration values in the script.

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