Adding CF steepness parameter

[tapios]

Purpose

Adds a cloud fraction steepness fitting parameter.

To-do

Content

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• I have read and checked the items on the review checklist.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.78%. Comparing base (1e2cb53) to head (f74b0e1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #288   +/-   ##
=======================================
  Coverage   91.78%   91.78%           
=======================================
  Files           1        1           
  Lines         146      146           
=======================================
  Hits          134      134           
  Misses         12       12           

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