Add preliminary classes for version 2 of the postprocessing system

[joezuntz]

I plan to replace the post-processing system with something less confusing. This is the first step in that.

Also fixes a bug when emcee or MH chains are resumed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 1.70213% with 231 lines in your changes missing coverage. Please review.

Project coverage is 60.66%. Comparing base (5f92934) to head (60bffe1).
Report is 9 commits behind head on main.

Files	Patch %	Lines
cosmosis/postprocessing/v2/chains.py	0.00%	229 Missing ⚠️
cosmosis/output/text_output.py	80.00%	1 Missing ⚠️
cosmosis/postprocessing/v2/__init__.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
- Coverage   61.17%   60.66%   -0.52%     
==========================================
  Files         115      117       +2     
  Lines       11942    12207     +265     
==========================================
+ Hits         7306     7405      +99     
- Misses       4636     4802     +166     

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