Remove non standard NXpositioner groups

[noemifrisina]

First part of #195

This mid-way fix makes the non-standard groups optional and changes the NX_class from NXpositioner to NXtransformations

[codecov]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 44.04%. Comparing base (2a1593b) to head (ffa1050).
Report is 31 commits behind head on main.

❗ Current head ffa1050 differs from pull request most recent head 98ce56e. Consider uploading reports for the commit 98ce56e to get more accurate results

Files	Patch %	Lines
src/nexgen/nxs_write/NXclassWriters.py	90.62%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
+ Coverage   43.87%   44.04%   +0.16%     
==========================================
  Files          45       45              
  Lines        3893     3860      -33     
  Branches      742      732      -10     
==========================================
- Hits         1708     1700       -8     
+ Misses       2058     2040      -18     
+ Partials      127      120       -7     

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