Calculation.m: Add TODO to GetCalculationWhere
CodeGenCalculation.m: Move code closer to where it is used
CaKernel.m: Add an EXTERIOR option in cakernel.ccl to indicate whether a kernel should run on the exterior or not

Previously, this was determined by the STENCIL size, but this is different to how the host version works, and leads to different numerical results in ghost zones (e.g. for RHS functions which are not synchronised).