Add a very basic SMAP L2 soil moisture reader - code from Rolle

[adybbroe]

This is a very minimal first version for a SMAP L2 soil moisture reader

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

[codecov]

Codecov Report

Attention: Patch coverage is 47.05882% with 9 lines in your changes missing coverage. Please review.

Project coverage is 96.13%. Comparing base (17cf0ce) to head (49871aa).
Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/readers/smap_l2_smpe.py	47.05%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3083      +/-   ##
==========================================
- Coverage   96.14%   96.13%   -0.02%     
==========================================
  Files         383      384       +1     
  Lines       55785    55817      +32     
==========================================
+ Hits        53636    53659      +23     
- Misses       2149     2158       +9     

Flag	Coverage Δ
behaviourtests	3.88% <0.00%> (+<0.01%)	⬆️
unittests	96.22% <47.05%> (-0.02%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 13926493235

Details

• 9 of 18 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 96.238%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/readers/smap_l2_smpe.py	9	18	50.0%

Totals
Change from base Build 13922886695:	-0.02%
Covered Lines:	53907
Relevant Lines:	56014

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💛 - Coveralls