np.acos -> np.arccos

[keurfonluu]

• Fixed: issue due to np.acos being an alias of np.arccos introduced in Numpy v2.

Reminders:

• Run invoke ruff to make sure the code follows the style guide,
• Add tests for new features or tests that would have caught the bug that you're fixing,
• Write detailed docstrings for all functions, classes and/or methods,
• If adding new functionality, unit test it and add it to the documentation.