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 78.18182% with 36 lines in your changes missing coverage. Please review.

Project coverage is 95.99%. Comparing base (9e3b342) to head (ed528e3).
Report is 69 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/readers/thr3mi_l1c_nc.py	53.84%	36 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2908      +/-   ##
==========================================
- Coverage   96.06%   95.99%   -0.07%     
==========================================
  Files         373      375       +2     
  Lines       54465    54571     +106     
==========================================
+ Hits        52321    52388      +67     
- Misses       2144     2183      +39     

Flag	Coverage Δ
behaviourtests	3.98% <0.00%> (-0.01%)	⬇️
unittests	96.09% <78.18%> (-0.07%)	⬇️

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