warning fixes in Mvd

The MR fixes the following warnings (that can be seen here: https://cdash.gsi.de/buildSummary.php?buildid=432309):

===

/home/cbmdock/cbmroot/core/detectors/mvd/CbmMvdDetector.cxx: In member function 'TH1* CbmMvdDetector::GetHistogram(UInt_t, UInt_t, UInt_t)':
/home/cbmdock/cbmroot/core/detectors/mvd/CbmMvdDetector.cxx:398:42: warning: unused parameter 'nLevel' [-Wunused-parameter]
 TH1* CbmMvdDetector::GetHistogram(UInt_t nLevel, UInt_t nHistogramNumber, UInt_t sensorInSensorArrayNumber)
                                   ~~~~~~~^~~~~~
/home/cbmdock/cbmroot/core/detectors/mvd/CbmMvdDetector.cxx:398:57: warning: unused parameter 'nHistogramNumber' [-Wunused-parameter]
 TH1* CbmMvdDetector::GetHistogram(UInt_t nLevel, UInt_t nHistogramNumber, UInt_t sensorInSensorArrayNumber)
                                                  ~~~~~~~^~~~~~~~~~~~~~~~
/home/cbmdock/cbmroot/core/detectors/mvd/CbmMvdDetector.cxx:398:82: warning: unused parameter 'sensorInSensorArrayNumber' [-Wunused-parameter]
 TH1* CbmMvdDetector::GetHistogram(UInt_t nLevel, UInt_t nHistogramNumber, UInt_t sensorInSensorArrayNumber)
                                                                           ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

===

In file included from /home/cbmdock/cbmroot/core/detectors/mvd/SensorDataSheets/CbmMvdSensorDataSheet.cxx:10:
/home/cbmdock/cbmroot/core/detectors/mvd/SensorDataSheets/CbmMvdSensorDataSheet.h: In constructor 'CbmMvdSensorDataSheet::CbmMvdSensorDataSheet()':
/home/cbmdock/cbmroot/core/detectors/mvd/SensorDataSheets/CbmMvdSensorDataSheet.h:65:43: warning: 'CbmMvdSensorDataSheet::fSignalFallTime' will be initialized after [-Wreorder]
   Double_t fSignalFallTime  = nan("NotSet");
                                           ^
/home/cbmdock/cbmroot/core/detectors/mvd/SensorDataSheets/CbmMvdSensorDataSheet.h:61:12: warning:   'Double_t CbmMvdSensorDataSheet::fShaperNormalisationFactor' [-Wreorder]
   Double_t fShaperNormalisationFactor;

/home/cbmdock/cbmroot/core/detectors/mvd/SensorDataSheets/CbmMvdSensorDataSheet.cxx:21:1: warning:   when initialized here [-Wreorder]
 CbmMvdSensorDataSheet::CbmMvdSensorDataSheet()
 ^~~~~~~~~~~~~~~~~~~~~

/home/cbmdock/cbmroot/core/detectors/mvd/SensorDataSheets/CbmMvdSensorDataSheet.cxx: In member function 'virtual Double_t CbmMvdSensorDataSheet::ComputeCCE(Float_t, Float_t, Float_t, Float_t, Float_t, Float_t)':
/home/cbmdock/cbmroot/core/detectors/mvd/SensorDataSheets/CbmMvdSensorDataSheet.cxx:104:96: warning: unused parameter 'chargePointZ' [-Wunused-parameter]
 Double_t CbmMvdSensorDataSheet::ComputeCCE(Float_t chargePointX, Float_t chargePointY, Float_t chargePointZ,
                                                                                        ~~~~~~~~^~~~~~~~~~~~
/home/cbmdock/cbmroot/core/detectors/mvd/SensorDataSheets/CbmMvdSensorDataSheet.cxx:105:84: warning: unused parameter 'diodeZ' [-Wunused-parameter]
                                            Float_t diodeX, Float_t diodeY, Float_t diodeZ)
                                                                            ~~~~~~~~^~~~~~

===

/home/cbmdock/cbmroot/sim/detectors/mvd/CbmMvdDigitizer.cxx: In member function 'void CbmMvdDigitizer::DisplayDebugHistos()':
/home/cbmdock/cbmroot/sim/detectors/mvd/CbmMvdDigitizer.cxx:429:12: warning: variable 'c' set but not used [-Wunused-but-set-variable]
   TCanvas* c  = 0;

===

[CTest: warning suppressed] /Users/fairroot/builds/computing/cbmroot/sim/detectors/mvd/CbmMvdDigitizer.cxx:443:41: warning: adding 'char' to a string does not append to the string [-Wstring-plus-int]
    TString myCanvasName = "DigiCanvas" + myChar;
[CTest: warning suppressed]                            ~~~~~~~~~~~~~^~~~~~~~
[CTest: warning matched] /Users/fairroot/builds/computing/cbmroot/sim/detectors/mvd/CbmMvdDigitizer.cxx:443:41: note: use array indexing to silence this warning
    TString myCanvasName = "DigiCanvas" + myChar;
[CTest: warning suppressed]                                         ^
                           &            [       ]
[CTest: warning suppressed] 1 warning generated.

        ^

===

[CTest: warning matched] /Users/fairroot/builds/computing/cbmroot/reco/detectors/mvd/plugins/tasks/CbmMvdSensorHitfinderTask.cxx:310:10: warning: variable 'shape' set but not used [-Wunused-but-set-variable]
  UInt_t shape  = 0;
[CTest: warning suppressed]          ^
[CTest: warning suppressed] 1 warning generated.

/home/cbmdock/cbmroot/reco/detectors/mvd/plugins/tasks/CbmMvdSensorHitfinderTask.cxx: In member function 'void CbmMvdSensorHitfinderTask::ComputeCenterOfGravity(CbmMvdCluster*, TVector3&, TVector3&, double&, double&, double&)':
/home/cbmdock/cbmroot/reco/detectors/mvd/plugins/tasks/CbmMvdSensorHitfinderTask.cxx:311:9: warning: variable 'xIndex0' set but not used [-Wunused-but-set-variable]
   Int_t xIndex0 = 0;
         ^~~~~~~
/home/cbmdock/cbmroot/reco/detectors/mvd/plugins/tasks/CbmMvdSensorHitfinderTask.cxx:312:9: warning: variable 'yIndex0' set but not used [-Wunused-but-set-variable]
   Int_t yIndex0 = 0;
         ^~~~~~~