Changes needed for big distortions.

Changes needed for big distortions.
+
Example usage of the functionality spaceChargeSimulationTrack.C:DrawFuncionIntegralSpaceCharge()

Marian

Using new functionality in base class:
a.)  Step towards proper treatment of non commutativness combining several
distortion
b.)  Fix of the non linearities of the AliTPCSpaceCharge3D

Function to be  added (see attached diff file):

1. Derivative of correction
         virtual void GetCorrectionDz(const Float_t x[],const Short_t
roc,Float_t dx[], Float_t delta);
2. Integral of the distortion along drift line:
          virtual void GetCorrectionIntegralDz(const Float_t x[],const Short_t
roc,Float_t dx[], Float_t delta);
+
corresponding visualisation functions:
+  static Double_t GetCorrXYZDz(Double_t gx, Double_t gy, Double_t gz, Int_t
axisType, Int_t corrType=0,Double_t delta=5);
+  static Double_t GetCorrXYZIntegrateZ(Double_t gx, Double_t gy, Double_t gz,
Int_t axisType, Int_t corrType=0, Double_t delta=5);