I added silver stepsizes to PEPit

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 88.77%. Comparing base (ce62423) to head (5ba56f6).

:exclamation: Current head 5ba56f6 differs from pull request most recent head e1dff91

Please upload reports for the commit e1dff91 to get more accurate results.

Files	Patch %	Lines
...ization/gradient_descent_silver_stepsize_convex.py	87.87%	4 Missing :warning:
...radient_descent_silver_stepsize_strongly_convex.py	92.15%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #96 +/- ## ========================================== - Coverage 90.24% 88.77% -1.47% ========================================== Files 171 173 +2 Lines 6364 6460 +96 ========================================== - Hits 5743 5735 -8 - Misses 621 725 +104 ```

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