Fix GPU b-field initialization

The 12kA L3 field should be scaled wrt its own nominal rather than 30kA nominal Bz.
Keep the old scaling for backward compatibility

Author: shahor02

GPU/GPUTracking/Interface/GPUO2InterfaceUtils.cxx

@@ -142,3 +142,9 @@ uint32_t GPUO2InterfaceUtils::getTpcMaxTimeBinFromNHbf(uint32_t nHbf)
 {
   return (nHbf * o2::constants::lhc::LHCMaxBunches + 2 * o2::tpc::constants::LHCBCPERTIMEBIN - 2) / o2::tpc::constants::LHCBCPERTIMEBIN;
 }
+
+float GPUO2InterfaceUtils::getNominalGPUBzFromCurrent(float l3curr)
+{
+  float al3curr = CAMath::Abs(l3curr);
+  return (CAMath::Abs(al3curr - 12000) < CAMath::Abs(al3curr - 30000) ? (2.04487f / 12000.f) : (5.00668f / 30000.f)) * l3curr;
+}

GPU/GPUTracking/Interface/GPUO2InterfaceUtils.h

@@ -53,8 +53,9 @@ class GPUO2InterfaceUtils
   template <class T>
   static float getNominalGPUBz(T& src)
   {
-    return (5.00668f / 30000.f) * src.getL3Current();
+    return getNominalGPUBzFromCurrent(src.getL3Current());
   }
+  static float getNominalGPUBzFromCurrent(float l3curr);
   static std::unique_ptr<GPUParam> getFullParam(float solenoidBz, uint32_t nHbfPerTf = 0, std::unique_ptr<GPUO2InterfaceConfiguration>* pConfiguration = nullptr, std::unique_ptr<GPUSettingsO2>* pO2Settings = nullptr, bool* autoMaxTimeBin = nullptr);
   static std::shared_ptr<GPUParam> getFullParamShared(float solenoidBz, uint32_t nHbfPerTf = 0, std::unique_ptr<GPUO2InterfaceConfiguration>* pConfiguration = nullptr, std::unique_ptr<GPUSettingsO2>* pO2Settings = nullptr, bool* autoMaxTimeBin = nullptr); // Return owning pointer
   static void paramUseExternalOccupancyMap(GPUParam* param, uint32_t nHbfPerTf, const uint32_t* occupancymap, int32_t occupancyMapSize);