Use expm1 in n_thermal for stability.

[albertomercurio]

Checklist

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Description

In the n_thermal function we are currently computing y = exp(x) and then y - 1. This can be directly done with the expm1, which is specifically designed to handle the x <<1 case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.22%. Comparing base (6cb6ef8) to head (4c4e986).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #717   +/-   ##
=======================================
  Coverage   93.22%   93.22%           
=======================================
  Files          57       57           
  Lines        3912     3914    +2     
=======================================
+ Hits         3647     3649    +2     
  Misses        265      265           

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