Cleanup: Operator tests

[samuelburbulla]

Cleanup: Operator tests

Description

This pr introduces fixtures and functions to align the testing of all operators.

Which issue does this PR tackle?

• Fixes #91

How does it solve the problem?

• Introduces fixture containing the product of different dimensions (1 and 2) for all vectors in the dataset.
• Introduces a utility function that runs a set of datasets against a set of operators and checks if the output and the ground truth mapping have the same shape.

How are the changes tested?

• The new tests run for all operators already implemented (according to their properties).

Open Questions

• Not every operator is able to fulfill all properties that could be tested in an operator context. Therefore only a subset of these properties need testing for restricted operators (e.g. a fno is not domain independent).
• What properties should be tested?
  • in- and output space descriptions:
  • different dimensions in x and u (fixture tests this),
  • different number of dimensions in y and v (fixture tests this),
  • different number of dimensions in x and y (domain independence, fixture tests this),
  • different number of dimensions in u and v (domain independence, fixture tests this),
  • convergence on a trivial identity mapping,
  • Test for different number of evaluation points.
• Should these properties be able to be addressed individually?
• Should we introduce a documentation page that lists different properties (domain independence, discretization invariance, ...) of neural operators and categorizes them?

Checklist for Contributors

• [x] Scope: This PR tackles exactly one problem.
• [x] Conventions: The branch follows the feature/title-slug convention.
• [x] Conventions: The PR title follows the Bugfix: Title convention.
• [x] Coding style: The code passes all pre-commit hooks.
• [x] Documentation: All changes are well-documented.
• [x] Tests: New features are tested and all tests pass successfully.
• [x] Changelog: Updated CHANGELOG.md for new features or breaking changes.
• [x] Review: A suitable reviewer has been assigned.

Checklist for Reviewers:

• [x] The PR solves the issue it claims to solve and only this one.
• [x] Changes are tested sufficiently and all tests pass.
• [x] Documentation is complete and well-written.
• [x] Changelog has been updated, if necessary.