Main purpose was to have option for per wire tzero offsets

Modified THcDriftChamberPlane.h
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1) added fUsingTzeroPerWire as parameter flag to switch on per wire tzero offsets when parameter equals 1
2) fTzeroWire is array of tzero offsets for each wire in plane

Modified THcDriftChamberPlane.cxx
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1) THcDriftChamberPlane::ReadDatabase
    a)set a default value of fUsingTzeroPerWire=0
    b)added to DBRequest list option to read in fUsingTzeroPerWire
    c) if fUsingTzeroPerWire=1 then separate DBRequest to read in fTzeroWire
   d) if fUsingTzeroPerWire=0 then all fTzeroWire=0

2) THcDriftChamberPlane::ProcessHits
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   a) Modify calcualation of drift time to subtract fTzeroWire
Double_t time = -StartTime- rawtdc*fNSperChan + fPlaneTimeZero - fTzeroWire[wireNum-1];