Update mass fluxes at end of timestep with full timestep's worth

[maxpkatz]

PR summary

This update makes the mass_fluxes array hold the full timestep's worth of mass fluxes at the end of finalize_advance. (This subtlety only matters if there were multiple subcycles in the timestep, in which case mass_fluxes would presently only have the last subcycle's worth of mass fluxes.)

PR motivation

This is a first step towards implementing the request in #1691.

PR checklist

• [x] 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 pass: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2022-04-28-005/index.html

[zingale]

tests pass: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2022-10-27-001/index.html