Cactus: New cGH entries cctk_level, cctk_block

Add new cGH entries cctk_level and cctk_block. These complement the existing cctk_patch and cctk_tile_{min,max} fields, and allow the compute kernels to know on which part of the grid structure they are operating. This is mostly interesting for producing better debugging output.

There are also new #defines CCTK_HAVE_CGH_LEVEL and CCTK_HAVE_CGH_BLOCK to indicate that these new fields are present.