Fix SLSTR angle interpolation.

[simonrp84]

This PR adds a new method for interpolating angles from the SLSTR tie grid, which should prevent the unrealistically low and/or high values that sometimes showed up before, primarily in granules near the poles.

• Closes extrapolation of satellite relative azimuiths? #3015
• Fully documented

[codecov]

Codecov Report

Attention: Patch coverage is 27.27273% with 8 lines in your changes missing coverage. Please review.

Project coverage is 96.11%. Comparing base (869e4fe) to head (f770395).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/readers/slstr_l1b.py	27.27%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3017      +/-   ##
==========================================
- Coverage   96.13%   96.11%   -0.02%     
==========================================
  Files         383      383              
  Lines       55688    55697       +9     
==========================================
+ Hits        53534    53535       +1     
- Misses       2154     2162       +8     

Flag	Coverage Δ
behaviourtests	3.88% <0.00%> (-0.01%)	⬇️
unittests	96.21% <27.27%> (-0.02%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 13543319156

Details

• 3 of 11 (27.27%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 96.223%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/readers/slstr_l1b.py	3	11	27.27%

Totals
Change from base Build 13543289700:	-0.01%
Covered Lines:	53782
Relevant Lines:	55893

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💛 - Coveralls

[mraspaud]

Interesting method, thanks for implementing it! How about adding a test or two for it?