Small fix to CO core masses for AGB stars

[TomWagg]

In #756 we changed the conditions in assign_remnant.f to use mc_co instead of mc. However, we didn't ensure that mc_co was set to mcmax if we overshot it. This meant that there was a timestepping issue that gave you a nonmontonic result of an SN occuring with smooth changes in initial masses and metallicity.

Here's the testing script:
min_sn_crit.py

Before:

After:

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.83%. Comparing base (8772c07) to head (78f87a7).
⚠️ Report is 137 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/cosmic/_version.py	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (50.00%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #762      +/-   ##
===========================================
- Coverage    86.91%   77.83%   -9.08%     
===========================================
  Files           40       50      +10     
  Lines        25542    27517    +1975     
  Branches         0      921     +921     
===========================================
- Hits         22198    21417     -781     
- Misses        3344     5813    +2469     
- Partials         0      287     +287     

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[katiebreivik]

Thanks @TomWagg !!