Updated DC Classes to have DC Sigma per wire

1) THcDCWire.h
  a) added sigma to THcDCWire constructor which fills fSigmaWire
  b) added GetSimga method
  c) added SetSigma method

2) THcDCHit.h
 a) Added GetWireSigma method

3) THcDriftChamberPlane.h
 a) Added flag fUsingSigmaPerWire
 b) Added array fSigmaWire

4) THcDriftChamberPlane.cxx
 a) In ReadDatabase, set fUsingSigmaPerWire=0 by default
    with reading of parameter h_using_sigma_per_wire optional
    for setting fUsingSigmaPerWire.
 b) If fUsingSigmaPerWire=0 then sets all wires in array fSigmaWire to
   fSigma ( which is the simga per plane).
 c) If fUsingSigmaPerWire=1 then reads in the sigma per wire from
   a parameter file.
 d) Used new THcDCWire constructor to set the sigma for wire to fSigmaWire[nwire]
 e) Also change code so that the tzero offset is set in the  THcDCWire constructor
 f) In ProcessHits , use wire->GetTOffSet instead of the fTzeroWire array when setting
    the time.

5) THcDC.cxx
 a) In TrackFit, replace fSigma with hit->GetWireSigma()