Adding kernel for Newton-Schulz inverse

[asobczyk]

$ pytest -s tests/test_tri_inv_ns.py 
=============================================== test session starts ================================================
platform linux -- Python 3.10.12, pytest-8.3.4, pluggy-1.6.0
rootdir: /home/asobczyk/software/pto-kernels
configfile: pytest.ini
collected 400 items                                                                                                

tests/test_tri_inv_ns.py [W416 13:34:00.010460655 TensorFactories.cpp:338] Warning: Cannot create tensor with interal format while allow_internel_format=False, tensor will be created with base format. (function operator())
................................................................................................................................................................................................................................................................................................................................................................................................................

=============================================== 400 passed in 16.56s ===============================================

[gioelegott]

Tested locally and everything passes. I trust @asobczyk for the correct absolute and relative errors.

I left some minor comments, but feel free to merge.

[zouzias]

Nitpick for doxygen docs. I thought CI should complain about it :-(

[zouzias]

Minor nitpicks, feel free to merge.