class AliDigitizationInput: public TNamed

 AliDigitizationInput.cxx
 RS: Created from former AliDigitizationInput. It does not anymore manage digitization:
 (now it is steared by the AliSimulation) but just provides input info for
 AliDetector::SDigits2Ditits.

 Manager object for merging/digitization

 Instance of this class manages the digitization and/or merging of
 Sdigits into Digits.

 Only one instance of this class is created in the macro:
   AliDigitizationInput * manager =
      new AliDigitizationInput(nInputStreams,SPERB);
 where nInputStreams is number of input streams and SPERB is
 signals per background variable, which determines how combinations
 of signal and background events are generated.
 Then instances of specific detector digitizers are created:
   AliMUONDigitizer *dMUON  = new AliMUONDigitizer(manager)
 and the I/O configured (you have to specify input files
 and an output file). The manager connects appropriate trees from
 the input files according a combination returned by AliMergeCombi
 class. It creates TreeD in the output and runs once per
 event Digitize method of all existing AliDetDigitizers
 (without any option). AliDetDigitizers ask manager
 for a TTree with input (manager->GetInputTreeS(Int_t i),
 merge all inputs, digitize it, and save it in the TreeD
 obtained by manager->GetTreeD(). Output events are stored with
 numbers from 0, this default can be changed by
 manager->SetFirstOutputEventNr(Int_t) method. The particle numbers
 in the output are shifted by MASK, which is taken from manager.

 The default output is to the signal file (stream 0). This can be
 changed with the SetOutputFile(TString fn)  method.

 Single input file is permitted. Maximum MAXSTREAMSTOMERGE can be merged.
 Input from the memory (on-the-fly merging) is not yet
 supported, as well as access to the input data by invoking methods
 on the output data.

 Access to the some data is via gAlice for now (supposing the
 same geometry in all input files), gAlice is taken from the first
 input file on the first stream.

 Example with MUON digitizer, no merging, just digitization

  AliDigitizationInput * manager = new AliDigitizationInput(1,1);
  manager->SetInputStream(0,"galice.root");
  AliMUONDigitizer *dMUON  = new AliMUONDigitizer(manager);
  manager->Exec("");

 Example with MUON digitizer, merge all events from
   galice.root (signal) file with events from bgr.root
   (background) file. Number of merged events is
   min(number of events in galice.root, number of events in bgr.root)

  AliDigitizationInput * manager = new AliDigitizationInput(2,1);
  manager->SetInputStream(0,"galice.root");
  manager->SetInputStream(1,"bgr.root");
  AliMUONDigitizer *dMUON  = new AliMUONDigitizer(manager);
  manager->Exec("");

 Example with MUON digitizer, save digits in a new file digits.root,
   process only 1 event

  AliDigitizationInput * manager = new AliDigitizationInput(2,1);
  manager->SetInputStream(0,"galice.root");
  manager->SetInputStream(1,"bgr.root");
  manager->SetOutputFile("digits.root");
  AliMUONDigitizer *dMUON  = new AliMUONDigitizer(manager);
  manager->SetNrOfEventsToWrite(1);
  manager->Exec("");