$Header$

Things to do on this thorn:
- make casts from datatype to CCTK_REAL explicit in
  GeneralizedPolynomial-Uniform fetch routines
- clean up the horrible inefficiency of the
     GeneralizedPolynomial-Uniform/Hermite interpolator
  (cf Erik Schnetter's E-mails and CactusBase/1366)


- add the old "uniform cartesian" interpolator (by Paul Walker & Thomas Radke)
  to CCTK_InterpLocalUniform()

- 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?