Add activation function after first layer

[MLuchmann]

Bugfix: Add activation function after first layer

Description

No activation function was applied after the first layer in the residual network. Also, the actual width of a network was given by width+1. Now, it correctly correspond to the width parameter.

To make the test scripts consistent to previous tests, the widths had to be adjusted by +1. Without this fix, the tests failed

Checklist for Contributors

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

Checklist for Reviewers:

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

[samuelburbulla]

The tests failed because the activation has to be applied even for depth = 1 (otherwise we have no universal approximation properties). I changed this and therefore was able to remove the changes in the tests. They now use smaller networks but as they still work, it's fine. Also added an assertion to make sure nobody uses the DeepResidualNetwork with depth = 0.