change cGroup centeringtable into flat list of integers

[Roland Haas]

Currently, and for mostly historical reason, since it started out as a tag, centeringtable is a handle making its use less convenient than may be desired. For the most part this data is used by the driver, since user codes normally “know” what centering their grid functions have. However it may also be used by non-driver infrastructure thorns.

Also it may be good to define a default centering Cactus wide instead of letting the driver define its defaults so that non-driver infrastructure thorns can be provided with a centering value that matches what the driver uses if none is provided.

A more convenient data structure would be just a const int * similar to e.g. cGroupDynamicData.lsh. This would require that CCTK_GroupCenteringTableI uses a calling syntax similar to CCTK_GroupbboxGI ie

int CCTK_GroupCenteringTableGI(const cGH *cctkGH, int size, int *centeringtable, int groupindex);

which copies the data into a user supplied buffer to support C and Fortran callers.

While in the flesh all centering code is still considered experimental (and intentionally not documented yet) so can be changed.