Thr3mi L1C reader

[pepephillips]

• Closes #xxxx
• Tests added
• Fully documented

[pepephillips]

i just looked at the commits and there is an old MAP L1C reader commit in there. The MAP L1C readers were since removed and obviously not part of the final commit, only the new 3MI readers are there.

[codecov]

Codecov Report

Attention: Patch coverage is 76.96970% with 38 lines in your changes missing coverage. Please review.

Project coverage is 96.07%. Comparing base (0a734ba) to head (6f39755).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/readers/thr3mi_l1c_nc.py	53.84%	36 Missing ⚠️
satpy/tests/reader_tests/test_thr3mi_l1c_nc.py	97.70%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2908      +/-   ##
==========================================
- Coverage   96.13%   96.07%   -0.06%     
==========================================
  Files         383      385       +2     
  Lines       55673    55838     +165     
==========================================
+ Hits        53519    53646     +127     
- Misses       2154     2192      +38     

Flag	Coverage Δ
behaviourtests	3.87% <0.00%> (-0.02%)	⬇️
unittests	96.16% <76.96%> (-0.06%)	⬇️

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