clean up the artifical viscosity routines

[zingale]

we just need to compute divu on the face once
this is prep for limiting the artifical viscosity coefficient for species conservation

PR summary

PR motivation

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 is prep for #2228

[zingale]

quick tests pass: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2024-07-28/index.html