Ensure random seeds are unique for multiprocessed sampling

[TomWagg]

The title tells you what's up mostly, I messed up the sampling in v3.7.6 because multiprocessing workers need to be initialised with their own random seeds. I offset each worker by the process ID now so all should be fine. I've also added a test to ensure this doesn't happen again :)

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.47%. Comparing base (8772c07) to head (6bb1294).
⚠️ Report is 176 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/cosmic/sample/sampler/independent.py	0.00%	4 Missing ⚠️
src/cosmic/_version.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #807       +/-   ##
============================================
- Coverage    86.91%   76.47%   -10.44%     
============================================
  Files           40       59       +19     
  Lines        25542    28046     +2504     
  Branches         0      919      +919     
============================================
- Hits         22198    21447      -751     
- Misses        3344     6295     +2951     
- Partials         0      304      +304     

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[katiebreivik]

👍