Only do reactions on leaf zones when not subcycling

[maxpkatz]

PR summary

As a follow-up to #2505, for non-subcycling advances we can skip burning if we're not on a "leaf" zone (i.e. a zone not covered by any fine levels). These zones will be updated by an avgDown() in post_timestep().

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]

this has a bounds issue:
http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2023-07-10-001/wdmerger_collision-nosubcycle.html

[maxpkatz]

Fixed for now by only masking on the valid zones. In a future commit we could possibly expand the functionality of makeFineMask to support ghost zones, or to be clever about it and decide whether to burn in the ghost zone based on whether the nearest valid zone is masked out.

[zingale]

tests: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2023-07-10-002/index.html