Adding AliITSDigitizer class to do merging and digitization . Based on the

Adding AliITSDigitizer class to do merging and digitization . Based on the
TTask method. AliITSDigitizer class added to the Makefile and ITSLinkDef.h
file. The following files required minor changes. AliITS, added functions
SetHitsAddressBranch, MakeBranchInTreeD and modified MakeBranchD.
AliITSsimulationSDD.cxx needed a Tree indepenent way of returning back to
the original Root Directory in function Compress1D. Now it uses gDirectory.