24.0-coverity-TrigT2BeamSpot

Do compile-time calculation of index in T2TrackBSLLPoly.cxx. the idx method is moved to a free function and made compile-time. The previous (runtime) implementation returned -1 for arguments out of an acceptable range (which is then used as an index into an array), now it will refuse to compile. The original runtime method is moved to testing code for comparison, to ensure compliance with the original.