Add reader for CAMEL emissivity datasets.

[simonrp84]

This PR adds a reader for the NASA / USGS CAMEL emissivity product, which is a Level 3 global, gridded, dataset of IR emissivity values at 13 spectral tie points.

As well as the emissivity data, this reader also supports the ancillary datasets (ndvi, quality flags, etc).

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.09%. Comparing base (5984c29) to head (d1affa6).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3023      +/-   ##
==========================================
- Coverage   96.10%   96.09%   -0.01%     
==========================================
  Files         377      379       +2     
  Lines       55163    55277     +114     
==========================================
+ Hits        53012    53118     +106     
- Misses       2151     2159       +8     

Flag	Coverage Δ
behaviourtests	3.93% <0.00%> (-0.01%)	⬇️
unittests	96.18% <100.00%> (-0.01%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 12544590386

Details

• 115 of 115 (100.0%) changed or added relevant lines in 2 files are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.006%) to 96.2%

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Files with Coverage Reduction	New Missed Lines	%
satpy/tests/utils.py	2	93.16%
satpy/tests/reader_tests/gms/test_gms5_vissr_l1b.py	3	98.67%
satpy/tests/reader_tests/gms/test_gms5_vissr_navigation.py	3	97.18%

Totals
Change from base Build 12299617024:	-0.006%
Covered Lines:	53363
Relevant Lines:	55471

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💛 - Coveralls

[pnuu]

Some small comments inline.

[simonrp84]

Thanks @pnuu! I've committed some changes that should address your comments.