ALICE3: avoid code duplication to get the b field in different devices

Author: mpuccio

Detectors/Upgrades/ALICE3/GlobalReconstruction/workflow/include/ALICE3GlobalReconstructionWorkflow/MagneticFieldHelper.h

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+// Copyright 2019-2020 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+#ifndef O2_ALICE3_GLOBALRECONSTRUCTION_MAGNETICFIELD_HELPER_H
+#define O2_ALICE3_GLOBALRECONSTRUCTION_MAGNETICFIELD_HELPER_H
+
+namespace o2::trk
+{
+
+void ensureAlice3Field(float bz);
+
+} // namespace o2::trk
+
+#endif

Detectors/Upgrades/ALICE3/GlobalReconstruction/workflow/src/MagneticFieldHelper.cxx

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+// Copyright 2019-2020 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+#include "ALICE3GlobalReconstructionWorkflow/MagneticFieldHelper.h"
+
+#include "DetectorsBase/Propagator.h"
+#include "Field/MagFieldParam.h"
+#include "Field/MagneticField.h"
+#include "Framework/Logger.h"
+
+#include <TGeoGlobalMagField.h>
+
+namespace o2::trk
+{
+
+void ensureAlice3Field(float bz)
+{
+  auto* globalField = TGeoGlobalMagField::Instance();
+  if (globalField->GetField() == nullptr) {
+    auto* field = new o2::field::MagneticField("ALICE3Mag", "ALICE 3 Magnetic Field", bz / 5.f, 0.0, o2::field::MagFieldParam::k5kGUniform);
+    globalField->SetField(field);
+    globalField->Lock();
+    LOGP(info, "Installed ALICE3 magnetic field with Bz={} kG", bz);
+  }
+  auto* propagator = o2::base::Propagator::Instance(false);
+  propagator->updateField();
+  propagator->setNominalBz(bz);
+}
+
+} // namespace o2::trk