Modified THcSpacePoint.h to have public member that keeps track
   of the spacepoint ID number for each chamber.
   Used in THcDC::LinkStubs

Modified THcDC.cxx and h
1) Add tree variables sp1_id and sp2_id which are the integer
    ID in the spacepoints in each chamber used in the golden track.
     The ID refers to index in array of spacepoints created in
     THcDriftChamber.cxx

2) Modify LinkStub to fill sp1_id and sp2_id for each track

3) For golden track fill sp1_ID_best and sp2_ID_best