Decouple MpdKalmanFilter from runReco.C

EDIT 2023-12-22: MpdKalmanFilter is so strongly coupled to whole code, it is pretty much impossible to decouple it in one go. Nevertheless, it is possible to partially isolate it by:

• get rid of its Init() method that needs FairRootAna class to read the magnetic field (terrible design) and does not do anything else
• get rid of its FairTask parent, as the class is a library that should be explicitely called by FairTasks which are dependent on it
• at present, it is pretty much impossible to get rid of its singleton nature. However there are actions which (hopefully) will keep it under control: 1. remove duplicate singleton calls from every class using it, by storing it in a member pointer 2. Make the dependency explicit in constructor of each reco.C FairTask using it, by passing *(MpdKalmanFilter::Instance()) as an argument

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See #113 (closed)

MpdKalmanFilter should not be initialized as a FairTask itself (as by its nature it is not), it should be loaded as a dependency for FairTask that needs it.

Probable culprit

InitStatus MpdKalmanFilter::Init()
{

   cout << "InitStatus MpdKalmanFilter::Init\n\n";

   // Check if mag. field was set - if not create the default

   fMagField = FairRunAna::Instance()->GetField();

   if (!fMagField || TMath::Abs(fMagField->GetBz(0, 0, 0)) < 0.01) {
      cout << " -I- Using the default constant magnetic field Bz = 5 kG " << endl;
      fMagField            = new MpdMultiField();
      MpdConstField *field = new MpdConstField();
      field->SetField(0., 0., 5.);                            // values are in kG
      field->SetFieldRegion(-230, 230, -230, 230, -375, 375); // cm
      ((MpdMultiField *)fMagField)->AddField(field);
      FairRunAna::Instance()->SetField(fMagField);
      fMagField->Init();
      cout << " -I- The magnetic field at (0,0,0) = (" << fMagField->GetBx(0, 0, 0) << "," << fMagField->GetBy(0, 0, 0)
           << "," << fMagField->GetBz(0, 0, 0) << ") kG" << endl;
   } else {
      cout << " -I- The magnetic field at (0,0,0) = (" << fMagField->GetBx(0, 0, 0) << "," << fMagField->GetBy(0, 0, 0)
           << "," << fMagField->GetBz(0, 0, 0) << ") kG" << endl;
      cout << "OVERHEAD FIELD INFO: " << ((MpdMultiField *)fMagField)->GetFieldList()->UncheckedAt(0) << " "
           << ((FairField *)((MpdMultiField *)fMagField)->GetFieldList()->UncheckedAt(0))->GetType() << endl;
   }

   fJacob.ResizeTo(5, 5);
   return kSUCCESS;
}