Add support for early stopping based on exceeding some threshold value

[maxpkatz]

PR summary

This was originally developed for wdmerger and Detonation, but now all applications should be able to use it.

amr.message_int is overridden to be 1 by default to support this.

PR checklist

• test suite needs to be run on this PR
• this PR will change answers in the test suite to more than roundoff level
• all newly-added functions have docstrings as per the coding conventions
• the CHANGES file has been updated, if appropriate
• if appropriate, this change is described in the docs

[zingale]

tests: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2022-05-20-002/index.html

I think the diffs are because it doesn't have all of the changes on development

[maxpkatz]

Merged with development

[zingale]

can you also update the docs here:
https://amrex-astro.github.io/Castro/docs/timestepping.html

[maxpkatz]

can you also update the docs here: https://amrex-astro.github.io/Castro/docs/timestepping.html

I could, but this isn't really a timestepping constraint, so it seems a little out of place.

[zingale]

okay. I just thought since at the top we list stopping criteria. But we should find somewhere else to doc this.