small things
  set centroid variables for drift correction
  read IO parameters to set (default) directory in which we write output files
  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)
  there should be an option to specify using the parameter-file initial
    guess at selected times (or cactus iteration numbers) instead of always
    using the AH posn from the previous time step -- this would be useful
    if there are an inner & outer horizon which start out coincident
    and you want to track them separately

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