Add function to retrieve sampled parameters by chains

[xgarrido]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.66667% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 79.22%. Comparing base (8cdd201) to head (3a99ea9).
Report is 1 commits behind head on master.

Files	Patch %	Lines
cobaya_utilities/tools.py	86.66%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
+ Coverage   78.67%   79.22%   +0.54%     
==========================================
  Files           4        4              
  Lines         408      438      +30     
==========================================
+ Hits          321      347      +26     
- Misses         87       91       +4     

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