Doxygen updates. Add \brief to many classes.

src/THcAerogel.cxx

src/THcAnalyzer.cxx

src/THcCherenkov.cxx

src/THcDC.cxx

src/THcDetectorMap.cxx

src/THcDummySpectrometer.cxx

src/THcFormula.cxx

src/THcHallCSpectrometer.cxx

src/THcHitList.cxx

src/THcHodoEff.cxx

src/THcHodoscope.cxx

src/THcInterface.cxx

src/THcParmList.cxx

src/THcRasteredBeam.cxx

src/THcRunParameters.cxx

src/THcScalerEvtHandler.cxx

src/THcScintillatorPlane.cxx

src/THcShower.cxx

src/THcShowerArray.cxx