This declaration in
cctki_GroupsOnGH.h is wrong; it should not have the
void *CCTKi_VarDataPtrI(const cGH *GH, int timelevel, int varindex) CCTK_ATTRIBUTE_PURE;
This function checks whether the caller has access to that grid variable and returns
NULL if not. It does not always return the same pointer.
Also, the variable pointer changes over time if the driver reallocates storage.