Interface + base class for equation modulus

[GiovanniCanali]

Description

This PR fixes #784

In this PR, the following updates have been introduced:

• equation Module:
  • Introduced a unified interface along with a base class structure for equations definition
  • Improved and expanded documentation for clarity and usability
  • Extended test coverage
• equation/zoo Submodule:
  • Moved all specific equation implementations to the new equation/zoo submodule
  • Standardized naming of source and test files in accordance with PINA conventions

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]

Waiting for PR #785 to be merged.