Refactor: optimize mesh usage

[pgibertini]

• Please check if the PR fulfills these requirements

• [x] The commit messages follows our guidelines (explicit, in english)

• [ ] Tests for the changes have been added (for bug fixes / features)

• [x] Docs have been added / updated (for bug fixes / features)

• [x] CHANGELOG.md has been updated

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• [ ] Bug fix:

• [ ] New features:

• [ ] Documentation

• [x] Optimization

• [x] Refactor

• Other information:

• Rename discrete_representation to discrete

• Rename cad_simplification to simplification

• Leverage Mesh3D class capabilities in simplification and voxelization

[pgibertini]

Create a class Shape instead of inheriting from Primitive3D

[GabrielJMS]

@pgibertini maybe the more appropriate name is FacetedModel/ FacetedShape. see: https://www.symscape.com/blog/faceted-modeling https://www.pre-scient.com/knowledge-center/geometric-modelling/faceted-modelling-and-nurbs/