improve integration of CarpetX interpolator with Cactus

[Roland Haas]

This adds a "CarpetX" local interpolator name to Cactus list of interpolators, so that client code can request an interpolator handle from CCTK_InterpHandle to pass to CCTK_InterpGridArrays. The actual handle is checked for and is the only one allowed. This prepares client code for a future full integration of the CarpetX interpolator with Cactus.

Pull request is here: https://github.com/eschnett/CarpetX/pull/248

[Roland Haas]

Please review.

[Erik Schnetter]

Merged as https://github.com/eschnett/CarpetX/commit/6b2acc45a9ac45b74b88fccf6c9cfbcca847557e .