Pylinkage

- Using Python 3.7 is officially deprecated ([end of life by 2023-06-27](https://devguide.python.org/versions/)).
It will no longer be tested, use it at your own risks!

- The optimizer tests are 5 times quicker (~1 second now) and raise less
false positive.
- The sidebar in the documentation makes navigation easier.
- A bit of reorganization in optimizers, it should not affect users.

- [matplotlib](https://matplotlib.org/) and tqdm now required.

- After many hours of computations, default parameters in
``particle_swarm_optimization`` are much more efficient. With the demo
``fourbar_linkage``, the output wasn't even convergent sometimes. Now we
have a high convergence rate (~100%), and results equivalent to the
``trials_and_errors_optimization`` (in the example).
- ``variator`` function of ``optimizer`` module was poorly working.
- The docstrings were not displayed properly in documentation, this is fixed.

- Examples moved from ``pylinkage/examples/`` to ``docs/examples/``.
- Tests moved from ``pylinkage/tests/`` to ``tests/``.

- Legacy built-in Particle Swarm Optimization, to avoid confusion.
- We do no longer show a default legend on static representation.