Add shekel funcfix

[ZohaibHassan16]

Description

This PR adds the Shekel function to the library's collection of standard algebraic test functions.

Related Issues

issue #10

Type of Change

• [ x] [ENH] - New feature (non-breaking change adding functionality))

How was this solved?

Implementation: Created shekel_function.py inside src/surfaces/test_functions/algebraic/standard/test_functions_nd/.

Registration: Updated the following init.py files to ensure the function is properly exported and accessible via the library's API:

src/surfaces/test_functions/algebraic/init.py

src/surfaces/test_functions/algebraic/standard/init.py

src/surfaces/test_functions/algebraic/standard/test_functions_nd/init.py

Checklist

Required

• [x ] PR title includes appropriate tag: [BUG], [ENH], [DOC] or [MNT]
• [x ] Code passes make check (lint, format, isort)

Tests

• [x ] Tests added/updated for changes

Documentation

• [ x] Documentation added/updated (docstrings, user guide, examples)

Note: New features ([ENH]) typically require both tests and documentation.
Bug fixes ([BUG]) should include a regression test when possible.

Testing

It can be verified by instantiating it through the standard algebraic interface:

from surfaces.test_functions.algebraic import ShekelFunction

func = ShekelFunction()
print(func.search_space)

Additional Notes

This is a fresh Pull Request intended to replace previous one that was "accidentally" combined with the other one

[SimonBlanke]

It would fit better to create a new module "test_functions_4d", because this test-function does not have a n_dim parameter. There is also some code-quality test, that fails.

[ZohaibHassan16]

Done and cleaned the code @SimonBlanke