Fix sampling when multiprocessing and using total_mass as your samplng target

[TomWagg]

This fixes an issue added in v3.7.6 where sample fails if you have total_mass as your sampling target and use multiprocessing.

[katiebreivik]

👍 thanks for the quick turnaround!

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.14%. Comparing base (8772c07) to head (a637b2a).
⚠️ Report is 173 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/cosmic/sample/sampler/independent.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #802       +/-   ##
============================================
- Coverage    86.91%   76.14%   -10.77%     
============================================
  Files           40       58       +18     
  Lines        25542    28171     +2629     
  Branches         0      985      +985     
============================================
- Hits         22198    21448      -750     
- Misses        3344     6418     +3074     
- Partials         0      305      +305     

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