Made debug static const in HelixArbitraryPlaneCrossing

To avoid complaints by the static analyzer, changed a static only
used for debugging to be const and made the version used only
with a conditional statement be thread safe.
Merge pull request #3757 from Dr15Jones/makeStaticConstHelixArbitraryPlaneCrossing

Reco -- Made debug static const in HelixArbitraryPlaneCrossing