GSF CubicInterpolation oscillation fix.

[The-Ludwig]

@pascalgutjahr noticed that in very rare cases, the GSF flux returns a close-to-zero negative value. It turned out this was due to the interpolation between the provided datapoints of the GSF model with scipy's CubicSpline.
As sugested here we switched to scipy's PchipInterpolator, which is monotone between the interpolated points and prevents the oscillations. It is still cubic.

The exact flux values changed a bit, so I had to recreate the test-values for GSF.

I added a test for this bug.

[github-actions]

Test Results

  6 files  ± 0    6 suites  ±0   2m 31s ⏱️ +8s
104 tests + 3  103 ✅ + 3  1 💤 ±0  0 ❌ ±0 
624 runs  +18  618 ✅ +18  6 💤 ±0  0 ❌ ±0 

Results for commit 4bc2756. ± Comparison against base commit ecb840d.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (ecb840d) to head (4bc2756).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           12        12           
  Lines          799       799           
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  Hits           799       799           

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