Add an option to log all failed parameter sets to a file

[joezuntz]

The hard part here is making this work under MPI without garbling the output. We use the MPI.File feature to do so, which seems to work, but should be considered experimental for a while until it has been tested at scale.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 79.36508% with 13 lines in your changes missing coverage. Please review.

Project coverage is 59.97%. Comparing base (4574989) to head (5e19527).

Files	Patch %	Lines
cosmosis/runtime/mpi_pool.py	65.38%	9 Missing ⚠️
cosmosis/main.py	42.85%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #135      +/-   ##
==========================================
+ Coverage   59.81%   59.97%   +0.15%     
==========================================
  Files         117      117              
  Lines       12248    12309      +61     
==========================================
+ Hits         7326     7382      +56     
- Misses       4922     4927       +5     

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