Change pocomc.Prior object to use a global Pipeline variabkle

[joezuntz]

Addresses issue #136

Currently the prior object cosmosis gives to PocoMC includes a reference to the complete pipeline object. If this fails to pickle then the PocoMC save state process fails too.

This changes things to use a global pipeline variable instead, which already existed because the sampler MPI needed it.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.96%. Comparing base (4574989) to head (921caa7).
Report is 3 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
+ Coverage   59.81%   59.96%   +0.15%     
==========================================
  Files         117      117              
  Lines       12248    12308      +60     
==========================================
+ Hits         7326     7381      +55     
- Misses       4922     4927       +5     

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