Cajita deprecation

[streeve]

Cajita was deprecated in #680. All users are encouraged to transition to using the new namespace, Cabana::Grid:: (replacing Cajita), and linking to the new library targets, Cabana::Grid (replacing Cabana::Cajita) and Cabana::Core (replacing Cabana::cabanacore).

All existing code should still work with the previous CMake targets and namespace. Warnings will be emitted on use of the old CMake targets (this can be disabled adding -Wno-dev to your CMake build options). Warnings will also be emitted for any usage of the Cajita namespace (this can be disabled by adding -DCabana_DISABLE_CAJITA_DEPRECATION_WARNINGS=ON to your Cabana CMake build options).

All backwards compatibility wrappers are intended to be removed in the next release of Cabana