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.

  If the podd docs are built using the ROOT tagfile, then when the hcana
  docs are built using the podd tag file, doxygen crashes.  This commit
  adds building of the podd docs to the hcana doc building Makefile.  The
  podd docs are built without using the ROOT tagfile, so a sed hack is used
  to modify podd's Doxyfile.

  Use ROOT's xml layout file
    Customize for hcana, with Hall C logo
  Use tag files from podd and ROOT
  Use groups, starting with a Detector group

  docs/Makefile now builds doxygen docs in docs/html
  If you want documentation to be build elsewhere, define
  DOXYGEN_OUTPUT_DIRECTORY and run doxygen by hand