small things
  set centroid variables for drift correction
  read IO parameters to set (default) directory in which we write output files
  it should be possible to set a mask to different values in the
    excised region, in the buffer zone between there & the AH,
    and outside the AH;  also it should be possible to supress
    setting the mask (i.e. just leave it unchanged) in any subset of these
  there should to be an option to make the excised region never shrink,
    i.e. if a point is ever excised then it would stay excised for the
    rest of the run
  add an option to compute/output the surface radius on a latitude/longitude
    grid
  I should set CXXFLAGS for this thorn to turn off automagic template
    instantiation on those platforms (eg lemieux, modi4) where it's on
    by default (Thomas Radke says to copy FlexIO's scheme for doing this)

medium things
  compute Gaussian curvature, cf AHFinder/src/AHFinder_gau.F
  HDF5 data files (simple format and/or Werner Benger's fancy one)
  move origin point to track moving BHs
  read AHFinder Fourier coeffs for initial guess
  write data files giving intersection of AH with a Cactus output plane
     (or more generally, with a hyperslab)
  export a "compute h(theta,phi)" function
     so other thorns could test point membership in/outside AH etc
  prevent h from going negative if the iteration isn't converging
     (maybe use log(h) in Newton iteration?)
  handle excision properly
     (detect when interpolator tries to use data from excised region
      and treat this as failure to evalute H(h) --> failure to find horizon
  checkpoint/recover doesn't work
     (needs storage of h gridfn in Cactus arrays,
      or else some equivalent mechanism)
  if two apparent horizons appear together, then move apart, right now
     even with N_horizons > 1 we will probably have each of our AHs track
     the same one; there should be some way to say "use AH 1 + something
     as the initial guess for AH2" or suchlike, so we could find both AHs

large things
  handle quadrant/octant grids with rotating BCs
     (needs Jacobian of one ghost zone to cover two different ghost zones)
  "isolated horizons" computation of BH mass and spin
  detect Cartoon and rotate geometry interpolation into Cartoon plane