feat: Add Noether (Nn) as a unit of momentum

[tilsammans]

This pull request introduces the 'Noether' (symbol: Nn) as a named unit for momentum, defined as 1 kg·m/s.

This proposal is formally discussed and advocated for in the American Journal of Physics article: 'It is time to honor Emmy Noether with a momentum unit' (https://pubs.aip.org/aapt/ajp/article/92/9/647/3309134/It-is-time-to-honor-Emmy-Noether-with-a-momentum). This gives the initiative strong academic backing.

• The unit 'noether = kilogram * meter / second = Nn' has been added to 'pint/default_en.txt'.
• A corresponding test case, 'test_noether_unit_conversion', has been added to 'pint/testsuite/test_quantity.py' to ensure the conversion works as expected.

This PR is meant as a grassroots initiative to get the unit more widely accepted. And also to settle on Nn. Both SciPy and Astropy refer to the SI and CODATA datasets, while the SI and official datasets won't include new units that are not widely used, presenting us with a catch-22.

[tilsammans]

To ensure this unit integrates seamlessly into the scientific Python ecosystem, I conducted a namespace and collision analysis across the SI, SciPy, Pint, and Astropy environments.

Why Nn was chosen over other symbols: I evaluated several candidates to find the most robust symbol that follows SI person-naming conventions (Capital-lowercase) while avoiding technical collisions:

• Nt: Rejected due to collision with the SI prefix/unit combination nT (nanotesla) and the obsolete symbol for Niton (Radon).
• No: Rejected due to conflict with the chemical element symbol for Nobelium. Also, Norway.
• Nr: Considered (as proposed in the AJP letter), but ultimately bypassed to avoid visual ambiguity with 'number' or 'near', and potential confusion with the Reynolds number in fluid dynamics.
• Nn (Selected): Fully machine-actionable and unique. It has no collisions with existing SI units or prefixes in SciPy, Pint, or Astropy. It provides a clean, distinct identifier that is easy to type and read.

Social & Educational Impact: Currently, all 19 SI units named after individuals honor men. The introduction of the Noether (Nn) provides a long-overdue opportunity to recognize Emmy Noether's fundamental contribution to physics—specifically the link between spatial translation symmetry and the conservation of momentum. This change is already being adopted by educators to improve clarity and student engagement.