AMReX-Astro/Castro: Release 20.07

The master branch has been renamed the main branch. If you have an existing clone of Castro, then do the following to update for this change. First, do git checkout master if you're not already on the old master branch. Then do git pull. This will gather the updates to the repo, but will fail with the message Your configuration specifies to merge with the ref 'refs/heads/master' from the remote, but no such ref was fetched. Then you can simply do git checkout main and your local repo should automatically switch to that branch and track updates from the upstream repo on GitHub. If you like, you can then delete the old master branch with git branch -D master. The CUDA build no longer has a requirement that amr.blocking_factor be a multiple of 8. Though this is recommended for performance reasons, it was previously required due to correctness reasons because of the use of an AMReX Fortran function, amrex_filccn. As noted in #1048, this function is no longer required due to recent changes in Castro (problems overriding bc_fill_nd.F90 or bc_ext_fill_nd.F90 do not need to provide an initial fill of the ghost zone data before implementing their specific boundary conditions; this is now done for you). Calling this function may now result in race conditions and correctness issues in the CUDA build, so it should be removed from any problem setups. (#1049) The functionality that permitted the rotation rate to change as a function of time, castro.rotation_include_domegadt and castro.rotational_dPdt, has been removed. (#1045) A CUDA illegal memory access error in Poisson gravity and diffusion has been fixed (#1039). The parameter castro.track_grid_losses has been removed. (#1035) The parameter castro.print_fortran_warnings, which no longer had any effect, has been removed. (#1036) PPM reconstruction has been added to the MHD solver (#1002) The Reactions_Type StateData has been reworked so that its first NumSpec components are rho omegadot rather than omegadot; then, the NumAux auxiliary components are stored, if the network has any auxiliary variables; then, rho enuc is stored (enuc itself is removed), and finally the burn weights are stored. The checkpoint version has been incremented, so this version of the code cannot restart from checkpoints generated with earlier versions of the code. (#927) A bug where refluxing between AMR levels resulted in incorrect results when a retry occurred in the previous timestep has been fixed. (#1018