Fix disable checkpointing

[mj-will]

Fixes a bug that prevented disabling the periodic checkpointing.

[codecov-commenter]

Codecov Report

Merging #101 (90f5ce5) into main (9362ba2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #101   +/-   ##
=======================================
  Coverage   94.30%   94.31%           
=======================================
  Files          39       39           
  Lines        3514     3515    +1     
=======================================
+ Hits         3314     3315    +1     
  Misses        200      200           

Flag	Coverage Δ
unittests	94.31% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nessai/nestedsampler.py	98.89% <100.00%> (+<0.01%)	⬆️

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