First public (open source) version of AthenaK. Fully working hydro an…

…d MHD (both non-relativistic and SR/GR), radiation, and Z4c solver for dynamical spacetimes. Both SMR and AMR. Tested on a wide variety of CPUs and GPUs. Major goals for next version is to get AMR working with radiation, and to get higher-order prolongation/restriction working with AMR for Z4c.

First working version in master (cpu only)