Set NumPy version requirement for Nutils cases

[MakisH]

Our Nutils cases implicitly depend on NumPy 1. NumPy 2 was recently released, which breaks such installations. This adds a numpy >1, <2 in all requirements.txt that also specify Nutils.

This brings up the issue that we should add requirements.txt for all other Python examples as well (in a separate PR). See #547

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md. -> N/A
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[IshaanDesai]

Looks good, but a cleaner solution would be to restrict the NumPy version in the Python bindings dependencies, and release a new version of the bindings. Adding the line in every tutorial is error prone and will increase maintainability.

pip would still update NumPy, even if we restrict it to a lower version at the bindings stage