Fix ABI L2 reader to produce reflectances as percentages

[djhoese]

As discussed on slack, this data handling has been broken for a long time (since creation?). Basically the CMIP Level 2 files are solar zenith correction versions of the L1b bands. Satpy expects reflectances to be 0 to ~100 (%), but in ABI files they are 0 to ~1. This PR fixes this.

CC @yufeizhu600

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

[codecov]

Codecov Report

Merging #2607 (7495e06) into main (3876475) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2607   +/-   ##
=======================================
  Coverage   94.94%   94.94%           
=======================================
  Files         354      354           
  Lines       51474    51487   +13     
=======================================
+ Hits        48873    48886   +13     
  Misses       2601     2601           

Flag	Coverage Δ
behaviourtests	4.26% <0.00%> (-0.01%)	⬇️
unittests	95.56% <100.00%> (+<0.01%)	⬆️

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

Files	Coverage Δ
satpy/readers/abi_l2_nc.py	96.22% <100.00%> (+0.22%)	⬆️
satpy/tests/reader_tests/test_abi_l2_nc.py	100.00% <100.00%> (ø)

[coveralls]

Pull Request Test Coverage Report for Build 6607378876

• 20 of 20 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 95.515%

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Totals
Change from base Build 6594974423:	0.001%
Covered Lines:	49009
Relevant Lines:	51310

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💛 - Coveralls

[mraspaud]

LGTM!

[djhoese]

I'm really not sure my "if" was the right thing here now. I'm only checking the units really, but plenty of L2 products have units of 1. I'll have to make another PR with something smarter.

[mraspaud]

Sorry I didn't spot it. I'm not familiar with the product...

[djhoese]

No. No problem. Just pointing out that I realized this after I went to bed last night. I'll clean it up shortly (hopefully).