$Header$

Things to do on this thorn:
- support excision, i.e. there should be a way to tell the interpolator
  about an excision mask, and either
  * have it return an error if the default-centering interpolator molecule
    would use data from an excised region, or preferably
  * properly implement the  excision_off_centering_tolerance
    and  excision_extrapolation_tolerance  parameters  as
    documented in the thorn guide

- 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?