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

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2763/improve-integration-of-carpatx