adapt multiple sensor reader to new L1g format from EUMETSAT

[ninahakansson]

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.31%. Comparing base (a246cc2) to head (0d5dcbd).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/readers/multiple_sensors_isccpng_l1g_nc.py	50.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3409   +/-   ##
=======================================
  Coverage   96.30%   96.31%           
=======================================
  Files         466      466           
  Lines       59088    59098   +10     
=======================================
+ Hits        56906    56918   +12     
+ Misses       2182     2180    -2     

Flag	Coverage Δ
behaviourtests	3.59% <0.00%> (-0.01%)	⬇️
unittests	96.40% <50.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.