fix shortwave underflows and mechred divide by zero

[apcraig]

[Remove this and add a short summary line]: Refactor code to eliminate shortwave underflow and mechred divide by zero.
Developer(s): ehunke, tcraig
Are the code changes bit for bit, different at roundoff level, or more substantial? bfb in base_suite, but could be non-bfb in some cases
Is the documentation being updated with this PR? (Y/N) N
If not, does the documentation need to be updated separately? (Y/N) N
"Documentation" includes information on the wiki and .rst files in doc/source/,
which are used to create the online technical docs at https://cice-consortium.github.io/Icepack/
Please suggest code reviewers in the column at right.
Other Relevant Details:

This fixes several debug abort on gnu

[apcraig]

This fixes 4 out of 6 failures on gnu with debug flags and addresses #48 and #118.

This also seems to be bit-for-bit in base suite testing. See 24755dd at https://github.com/CICE-Consortium/Test-Results/wiki/icepack_by_hash_forks

[eclare108213]

This looks fine to me. I'm surprised that the shortwave changes are BFB, since those were creating underflows.