Proposed changes

Allows to disable coarsening, meaning AMR will only ever refine or do nothing. One use case is to make sure multigrid levels created by AMR are guaranteed to be sequentially finer. Also useful to try AMR in BBH runs.

Upgrade instructions

In input files, add the following line to Amr.Policies to keep the current behavior unchanged:

AllowCoarsening: True

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments