[Users] Reorganisation of some of Carpet's internals

I have just re-organised some of Carpet's internal operators which implement the actual copy/restriction/prolongation operators. This will make it easier to implement new functionality that uses these operators, such as e.g. periodic boundary conditions within Carpet, or refluxing. The previous ad-hoc code was causing problems in some cases, and I hope that this cleaner re-implementation solves these.

The main change is to pass an additional argument "slabinfo" argument to the operators that can encode additional information, such as is needed e.g. for periodic symmetries or refluxing. In addition, the respective ad-hoc "fixes" have been removed.

-erik