Add classes of linear operators and quadratic functions

[NizarBousselmi]

Add the classes of symmetric and skew-symmetric linear operators and the class of homogeneous quadratic functions

[codecov[bot]]

Codecov Report

Attention: 106 lines in your changes are missing coverage. Please review.

Comparison is base (6b1a6c3) 89.72% compared to head (6e8118f) 88.37%. Report is 9 commits behind head on master.

Files	Patch %	Lines
PEPit/operators/linear.py	18.00%	41 Missing :warning:
...tions/smooth_strongly_convex_quadratic_function.py	25.00%	21 Missing :warning:
PEPit/operators/symmetric_linear.py	25.92%	20 Missing :warning:
PEPit/operators/skew_symmetric_linear.py	26.92%	19 Missing :warning:
PEPit/operators/cocoercive_strongly_monotone.py	88.88%	2 Missing :warning:
...EPit/functions/smooth_convex_lipschitz_function.py	92.30%	1 Missing :warning:
PEPit/functions/smooth_strongly_convex_function.py	0.00%	1 Missing :warning:
PEPit/operators/negatively_comonotone.py	92.85%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #85 +/- ## ========================================== - Coverage 89.72% 88.37% -1.36% ========================================== Files 151 158 +7 Lines 5012 5246 +234 ========================================== + Hits 4497 4636 +139 - Misses 515 610 +95 ```

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