Domain module refactoring

[GiovanniCanali]

Description

This PR fixes #574

Checklist

• Code follows the project’s Code Style Guidelines
• Tests have been added or updated
• Documentation has been updated if necessary
• Pull request is linked to an open issue

[GiovanniCanali]

This PR introduces the following improvements:

• Renames files to align with the PINA naming conventions (see Names convention #445).
• Separates the abstract interface from the base classes (see Adding base classes with only method definition #672).
• Implements the partial method for all domains and set operations, enabling full-boundary retrieval.
• Fixes uniform sampling for EllipsoidDomain and SimplexDomain, ensuring a truly uniform distribution.
• Redefines the sampling strategy for improved speed and robustness.
• Adds stricter, shared validation checks across all domains to prevent initialization issues.
• Expands the test suite for domains and set operations.
• Updates tutorial6.

These modifications do not affect the user experience, aside from the new additions.

[GiovanniCanali]

MEMO: if #717 is merged into dev before this PR, fix the conditions related to the spatial boundaries (use partial method).