fix: Linkage errors in BetheHeitlerApprox.hpp [backport #1777 to develop/v21.1.x]

[acts-project-service]

Backport a58b798 from #1777.

This should fix violations of ODR by either adding the inline keyword or by moving functions to a cpp file.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop/v21.1.x@e728d54). Click here to learn what that means.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##             develop/v21.1.x    #1800   +/-   ##
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  Lines                      ?    21066           
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  Hits                       ?    10213           
  Misses                     ?     4125           
  Partials                   ?     6728           

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[github-actions]

📊 Physics performance monitoring for 9b1cae3

🟥 ERROR The result has missing elements!
This is likely a physmon job failure

Full report
Seeding: ❌ seeded, ❌ truth estimated, ❌ orthogonal
CKF: seeded, truth smeared, truth estimated, ❌ orthogonal
IVF: seeded, truth smeared, truth estimated, ❌ orthogonal
Ambiguity resolution: seeded, ❌ orthogonal
Truth tracking
❌ Truth tracking (GSF)

Vertexing ❌

Vertexing vs. mu

IVF seeded

IVF truth_smeared

IVF truth_estimated

❌ IVF orthogonal

Seeding ❌

❌ Seeding seeded

❌ Seeding truth_estimated

❌ Seeding orthogonal

CKF ❌

CKF seeded

CKF truth_smeared

CKF truth_estimated

❌ CKF orthogonal

Ambiguity resolution

seeded

Truth tracking (Kalman Filter)

Truth tracking

Truth tracking (GSF)

Truth tracking

🖼️ gsf_plots/pull_d0.pdf ❌🖼️ gsf_plots/res_d0.pdf ❌🖼️ gsf_plots/pull_qop.pdf ❌🖼️ gsf_plots/res_qop.pdf ❌

[paulgessinger]

Failures are from intermediate fixes to the infra. I'm merging manually now.