Fix renaming old log_weight col correctly in importance sampler

[joezuntz]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 61.91%. Comparing base (5c5d1b5) to head (06852c5).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
cosmosis/samplers/importance/importance_sampler.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
- Coverage   61.94%   61.91%   -0.04%     
==========================================
  Files         117      117              
  Lines       12470    12473       +3     
==========================================
- Hits         7725     7723       -2     
- Misses       4745     4750       +5     

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