Modified THcDriftChamber

1) Add to tree variable arrays stub_x,stub_xp,stub_y and stub_yp
    and ncombos. These are arrays of the number of spacepoints in
    the chamber.

2) In ReadDatabase
   a) new variable  fRatio_xpfp_to_xfp which is set differently
       for HMS and SHMS. Used in method LeftRight
   b) Set default value of optional parameter fStubMaxXPDiff = 999.

3)  Modified method LeftRight
   a) Previously only for the old fHMSStyleChambers would
      the code only select LR combinations where
      the difference between stub_xp to space_point_X*ratio is
        with in the fStubMaxXPDiff .
      The ratio came from the HMS optics.
   b) Changed LeftRight so that if fStubMaxXPDiff < 100
         then the code will only select LR combinations where
      the difference between stub_xp to space_point_X*ratio is
        with in the fStubMaxXPDiff . The ratio is
      set in ReadDataBase according to the spectrometer.


5) In FindSpacePoints add some comments.