fix: Use exp_m1 instead of subtracting directly

[soumyasen1809]

• Added fix to use exp_m1 function to compute sf for Gumbel distribution more accurately

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.03%. Comparing base (a395dd1) to head (dccddfd).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #294   +/-   ##
=======================================
  Coverage   94.03%   94.03%           
=======================================
  Files          53       53           
  Lines       12053    12055    +2     
=======================================
+ Hits        11334    11336    +2     
  Misses        719      719           

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[YeungOnion]

figured I'd also add tests to make this a notable difference. These two tests fail the previous implementation and pass this one.