Feature/variable naming

[bgoujaud]

Factorize the way to define interpolation constraints on a class of functions. Add name to Points, Expressions, Functions and Constraints. Add automatic name to interpolation constraints. Add possibility to grab all the dual variables associated with interpolation constraints under the readable form of a tab.

Please merge PR 90 and 89 before this one.

[codecov-commenter]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (c576d35) 88.32% compared to head (46084a3) 90.41%.

Files	Patch %	Lines
PEPit/function.py	92.70%	7 Missing :warning:
PEPit/pep.py	73.91%	6 Missing :warning:
...trained_convex_minimization/gradient_descent_lc.py	82.14%	5 Missing :warning:
PEPit/functions/convex_support_function.py	81.25%	3 Missing :warning:
...convex_minimization/gradient_descent_quadratics.py	71.42%	2 Missing :warning:
...d_point_problems/inconsistent_halpern_iteration.py	83.33%	1 Missing :warning:
...examples/tutorials/gradient_descent_contraction.py	0.00%	1 Missing :warning:
PEPit/functions/smooth_strongly_convex_function.py	83.33%	1 Missing :warning:
PEPit/operators/negatively_comonotone.py	85.71%	1 Missing :warning:
PEPit/primitive_steps/inexact_proximal_step.py	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #91 +/- ## ========================================== + Coverage 88.32% 90.41% +2.08% ========================================== Files 167 171 +4 Lines 5910 6364 +454 ========================================== + Hits 5220 5754 +534 + Misses 690 610 -80 ```

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