Interface + base class for problem modulus

[GiovanniCanali]

Description

This PR fixes #783

In this PR, the following updates have been introduced:

• problem Module:
  • Introduced a unified interface along with a base class structure for problem definitions
  • Strengthened validation logic within AbstractProblem methods
  • Improved and expanded documentation for clarity and usability
  • Extended test coverage, including dedicated test files for each problem subclass (e.g., parametric, inverse, spatial, time-dependent)
• problem/zoo Submodule:
  • Standardized naming of source and test files in accordance with PINA conventions
  • Resolved minor issues in DiffusionReactionProblem and AcousticWaveProblem
  • Added solution-based tests wherever analytical solutions are available

The core logic underpinning the entire problem module has remained unchanged from a user perspective.

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]

Tests that are currently failing are unrelated to the changes introduced in this PR and will be addressed prior to the release of version 0.3.