Support built-in Equations in System of Equation

[GiovanniCanali]

Description

This PR fixes #592 .

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

[Copilot]

Pull Request Overview

This PR adds support for using built-in equation types (FixedValue, FixedGradient) inside a SystemEquation and updates tests accordingly.

• Tests now import and verify SystemEquation works with FixedValue and FixedGradient, for both constructor and residual
• Refactored SystemEquation.__init__ to build its internal list with a comprehension

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/test_equations/test_system_equation.py	Import and add tests for FixedValue/FixedGradient use cases
pina/equation/system_equation.py	Simplified list building of self.equations with isinstance

Comments suppressed due to low confidence (2)

pina/equation/system_equation.py:17

• The docstring still describes a single callable equation parameter. Please update the :param section to describe list_equation as a list of equations or callables (or EquationInterface instances) for clarity.

    def __init__(self, list_equation, reduction=None):

tests/test_equations/test_system_equation.py:76

• [nitpick] Currently only mean reduction is tested for built-in equations. Consider adding a sum reduction case to ensure both reducers work as expected with FixedValue and FixedGradient.

    res = system_eq.residual(pts, u)

[dario-coscia]

To me looks good!

Just one thing, in the documentation we should modify equation to also have Equation object and not just callables. Also an example in the doc would nice!

[GiovanniCanali]

To me looks good!

Just one thing, in the documentation we should modify equation to also have Equation object and not just callables. Also an example in the doc would nice!

Will do, thank you!