Add doxygen groups "Apparatuses" and "Decoders"
  Make sure every class is in a group
  Make sure most classes have a \brief description
  Improve comments for some classes including THcHallCSpectrometer
     and hodoscope classes
  Change some Doxyfile defaults.  Include sources files.

  Just replaces Process method.

Fix sign mistake in correction for vertical pointing offset
   in THcExtTarCor.cxx

Write Hall C version of THcReactionPoint