$Header$

Things to do on this thorn:
- HIGH PRIORITY: return interpolator status for each interpolated point:
    the user (optionally) provides in the parameter table, a pointer
    to a CCTK_INT array of size N_interp_points; we store the status there
    ==> this would allow Thomas Radke to fix CactusPUGH/1563
    ==> having both of these would allow me to fix a nasty AHFinderDirect bug
- make casts from datatype to CCTK_REAL explicit in
  GeneralizedPolynomial-Uniform fetch routines
- clean up the horrible inefficiency of the Hermite interpolator
  (cf Erik Schnetter's E-mails and CactusBase/1366)

- allow scalar  boundary_{off_centering,extrapolation}_tolerance
  values in the parameter table (to mean using this value for each ibndry)

- maybe change the defaults so off-centering is forbidden unless
  the user explicitly asks for it to be allowed?

- implement a "silent" parameter like in PUGHInterp?
  alternatively, print warning msgs for some error returns?