Add EMIT emit_l1b reader

[zxdawn]

• Tests added
• Fully documented

This PR supports reading the EMIT L1B NetCDF file provided by JPL.

[codecov]

Codecov Report

Merging #2592 (760ac6a) into main (f018f12) will increase coverage by 0.15%.
Report is 149 commits behind head on main.
The diff coverage is 34.06%.

@@            Coverage Diff             @@
##             main    #2592      +/-   ##
==========================================
+ Coverage   94.92%   95.07%   +0.15%     
==========================================
  Files         352      355       +3     
  Lines       51225    51361     +136     
==========================================
+ Hits        48624    48834     +210     
+ Misses       2601     2527      -74     

Flag	Coverage Δ
behaviourtests	4.23% <0.00%> (-0.05%)	⬇️
unittests	95.70% <34.06%> (+0.15%)	⬆️

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Files	Coverage Δ
satpy/readers/emit_l1b.py	34.06% <34.06%> (ø)

... and 75 files with indirect coverage changes

[coveralls]

Pull Request Test Coverage Report for Build 6448022511

• 25 of 68 (36.76%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.08%) to 95.414%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/readers/emit_l1b.py	25	68	36.76%

Totals
Change from base Build 6426189025:	-0.08%
Covered Lines:	48772
Relevant Lines:	51116

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💛 - Coveralls